I suspect, if it's a botnet, that whoever is using it has discovered that they can't mine full time and unload everything they mine without significantly pushing the market down. So, they might be mining for a while, until they collect some amount of bitcoins then they switch to something else lucrative while they unload the bitcoins over time...once finished unloading (and perhaps when done with the other activity) they go back to mining again to load back up on bitcoins.
Yep you are probably right. Funny how bitcoin is practically a botnet owner's dream - free money!