That's most likely the discrete graphics card kicking in. It's turned off unless it's needed, but when it turns on it puts out a lot of heat

Serious question: how long have you had the machine? If it's more than a year or two you might want to consider opening it up and blowing out all the dust and fluff. I have a 2015 model that runs it just fine, but would really have struggled two weeks ago before I gave it a clean out. Feels like I just upgraded the thing.

I generally do this every year or two but recently I've been doing a lot of work on the house so it's been particularly dusty with debris from plasterboard, cutting timber, sanding, etc.

Late 2013. The fans rarely rev like this. Only with intense CPU activity. If it was happening every other day then I'd take your point.

I'd still crack it open and blow out the dust: you might be surprised by the results.