I remember once, when I had an unpatched computer overrun by hostile viruses that rendered it unusable, wondering if one of the botnet-type viruses that want to take over your computer and use it without being undetected would someday get smart enough to recognize other viruses and automatically remove their competition so that they could continue to silently infect you. Like a biological virus, one that kills the host is an ineffective one - they want to use the hosts resources while not being noticed. I wonder if this is something like that - virus writers starting to get smarter about cleaning up behind them and keeping out competing malware.