Ah, I misunderstood.

It varies because some people back out of mining, and other times they jump-in because they just made the dive into bitcoin.  Or they got delivery of an ASIC and want use it quickly before it's worthless.

If the price of BTC in their currency of choice is too low, they may give up.

If it's high enough, they start mining again.

There are many reasons for global hashrate fluctuations.