You don't have to risk anything. If the trend continues, at some point one would be able to rent the mining power required.

On top of it, if you are a small miner, and you can't afford capital investments required to keep up with latest mining hardware, while loosing market share, you can use your soon-osolete hardware to make a one last buck. A big buck this time.

One possibility would be to keep up and coming competitor currencies from gaining a foothold. The big ASIC mining farms bitcoin has won't work with some of the smaller currencies. The organizations running them have a lot of money, and don't necessarily want to see competition from less centralized mining. Destroying trust in a currency by executing a double spend attack on it could be one way to prevent that.

Any organization/individual/government who has enough money that they aren't interested in cashing out and is also interested in medium/large scale economic disruption.

China, Russia, UAE, USA, sci-fi terrorists, evil villains, illuminati etc. (I'm only mostly joking)