The Lord had promised the people of Israel that he would put the fear/dread of them upon the people of Canaan (see Exodus 15:16; Exodus 23:27; Deuteronomy 2:25; Deuteronomy 11:25). Joshua 9:24 confirms that the fear of Israel was upon the people of Gibeon. The fact that the other kings join together in an alliance is also a testimony to the fact that they are fearful of what Israel can do and now seek to protect themselves. Racially diversified former enemies join in coalition against Israel.1
Some commentators suggest that the alliance instead shows that the Canaanite kings are no longer afraid of Israel and that is why they are now making plans to engage them in battle.2,3,4
2 they gathered together as one to fight against Joshua and Israel.