at what posts they were replies to.
Im not 100% sure the bots post as they first trawl.
With the 'deleted by poster' messages I think they may have already trawled and picked up where they were going to post before those messages were deleted. Otherwise it does seem strange, how, on the club bb for example, they would post to messages 6 months old.
Of course there may actually be no pattern at all, in the same way there is no pattern in the spam messages.

At one point, curious to see how many messages were being filtered out by the naughty words list, I emptied the list for one night. I got 1100 posts.
That was a couple of months ago.