Well ... if some pool wanted to destroy some of their smaller competition ...
It's quite common for people to hop off one pool onto another one when luck is bad but well within expectation.
Withholding on a non PPS pool will of course make the pool's luck seem worse.
Also there's the simple concept that people will rent hash due to wanting to gamble on that.
Renting hash will of course cost more than the expected BTC you will get, so renting is simply gambling.
Some gamblers can often be influenced by failure, to give it ... just one more try ... to get back their losses.
So if they aren't getting blocks (or getting fewer than expected) then they may well want to rent more ...
A couple of hours ago the data-centre where my main pool server is, was under a major DDoS attack.
It wasn't directed at my pool, but at the large data-centre.
Yep people may do such things simply because they want to and can do them.
That's how it is in the real world ...