Hi,
I think I had that experience in the old days too. (Now I haven't played much with cpus to notice it).
I believe it could be related with path finding.
I have the suspicion that if the AI messes up with path finding the first time they send you an army, it keeps messing it up.
The load allows the AI to recompute all the path finding and as a result it wakes it up.
It is possible that you could wake up the AI by also chopping or attacking.
All the above are just a hunch and I do not really know the problem/solution.
I somewhere read that someone tackled with the original AI and implemented an alternative one.
I would be interested to know how! I would like tackle with that :-P
Usually, AI is just scripting but still it could be fun.
Cheers