Ephraim was upset that Gideon had not called them during the original mustering of the tribes in Judges 6:34–35 to fight the enemies of the East. We cannot tell whether this was an oversight on Gideon’s part, a deliberate snub, or a decision he made for strategic purposes, since Ephraim was located in an ideal place to cut off the retreating enemy, which is exactly the contribution Gideon eventually called on them to make. But they have clearly interpreted it as a snub.1
1 Then the men of Ephraim said to him, “What is this that you have done to us, not to call us when you went to fight against Midian?” And they accused him fiercely.