Summary: It seems that as soon as Gideon was dead that they put off the worship of Jehovah God and turned to worship foreign gods; particularly, the local Baal gods.
Gideon and his God Were Soon Forgotten By an Ungrateful Israel [Judges 8.33-8.35]
33 And it came to pass, as soon as Gideon was dead, that the children of Israel turned again, and went a whoring after Baalim, and made Baalberith their god.
34 And the children of Israel remembered not the LORD their God, who had delivered them out of the hands of all their enemies on every side:
35 Neither shewed they kindness to the house of Jerubbaal, namely, Gideon, according to all the goodness which he had shewed unto Israel. –Judges 8:33-35 (KJV)
33 And it came to pass, as soon as Gideon was dead, that the children of Israel turned again, and went a whoring after Baalim, and made Baalberith their god.--Judges 8:33 (KJV)
33 As soon as Gideon died, the people of Israel chased after other gods—the Baals—as though they were prostitutes. They made Baal Berith their god.--Judges 8:33 (GW)
And it came to pass, as soon as Gideon was dead, that the children of Israel turned again,... from God, and the pure worship of Him, to idolatry: which reveals the disposition of the Hebrew people. It seems that as soon as Gideon was dead that they put off the worship of Jehovah God and turned to worship foreign gods; particularly, the local Baal gods. It was not an impulsive decision but was something they desired for a long time and perhaps even prepared for; they wanted a god like the nations around then, a God they could see and touch; and for a while (even while Gideon lived) they worshipped Gideon’s ephod; for which Gideon was to blame for allowing it to go on. And we should not be surprised that they chose Baal since they had worshipped him before. These people would have changed their allegiance long ago had they not been constrained by the presence and authority of their judges. There may have been overlapping periods of time between the Judges of Israel, but that was not the situation when Gideon died. No one was waiting in the wings that could replace Gideon; God would have to appoint someone.
The constant emphasis on fidelity to the Lord, which brings deliverance until the death of the judge, would seem to indicate that each leader’s judgeship was marked by a time of spiritual revival and renewed dedication to the Lord God of Israel. Tragically, however, the death of each judge was always followed by an increased period of idolatry and paganism. This time, the relapse into apostasy was especially severe in that the people made Baal-berith (see Article 8.7) their God. Berit is the Hebrew word for covenant. Their original covenant was with the Lord Jehovah, and this new act of rebellion caused them actually to recognize Baal as their new Lord of the covenant!
This is the same old story, is it not? The hoop of history continues to roll as it is rolling today. At first they were a nation who served God, and then they did evil, forsook God, turned to Baal, and God sells them into slavery and servitude. Then they cry out to God. Then they repent, and God raises up a judge to deliver them. Here goes Israel again. As soon as Gideon was dead, the children of Israel turned from God and went a–whoring after Baalim (see Article 8.6). That is the sad, sordid story of Israel, and also the story of His church today. This up and down business is the story of nations, churches, and individuals. Today many of us are just rolling a hoop through this world. One day we are up, and the next day we are down. God never intended our spiritual lives to be that way.
Article 8.6: Baalim
BAALIM: This was the general name that included all their idols. It is the plural of "Baal"; it occurs fifteen times in the Bible, but is not found in the Pentateuch or in the poetical books. The true significance of the word has been a matter of dispute. One of the leading explanations is that the expression is a "plural of majesty," equivalent to "the great god Ba'al," after the analogy of "Elohim" and "Adonim," which are two of the names of the one true God whom we worship. Apart from other objections that may be used against this point of view, it is said that such expressions always become proper names. Hence other explanations are more plausible; for example, that Baalim is images of the god Baal, or that they are the various forms in which Baal is worshiped. Since, however, there is no evidence of the formal worship in Israel of any Ba'al at a common center, and as the local Canaanite deities were known as the "baals" of their respective districts, and as Israel notoriously addicted itself to the cult of such deities, it is altogether probable that the expression designates the local deities to which such worship was paid in various places by the Hebrews in Palestine. Among other passages, Judges 8.33 is especially instructive on this point. In connection with the lapse of the people of Israel into the worship of the Baalim, it is there said that "they made Baal-berith their god" (see Baal-berith).