DoS attack more likely. When that happens the server gets maxed out by requests for pages or searches or in some cases new signups. That alone can crash lesser servers. On some the load is shifted off so a crash doesn't happen. Meantime the routers have picked up on the flood (of requests) and start to ignore any more till the level drops off below a certain threshold. Until the load drops off you get white pages and annoying error messages.
It's really bad if you're on shared hosting and it's another site getting pounded. Every one on that shared server gets to suffer in one form or another.