The way the AI works is that when war is declared it gathers it's dispered small units into gather points inorder to gather it's strength. If you happen to be sitting in the way or on it's gathering point It seem like it is throwing small armies/fleets at you. Is the location it chooses flawed, Yes. Should it organise before it DOW, Yes. Will it be fixed, NO.
BTW. I have seen very large AI fleets 100+
BTW. I have seen very large AI fleets 100+