My pH shot up this week

I've had a pretty stable pool all summer so far. pH has been at about 7.6 for 6 weeks.

But this morning I checked it and it's spiked to 8.0-plus. (Taylor kit).
Total alkalinity has climbed too, from about 110 to 140.

I'm not overly alarmed by it, a third gallon of muriatic acid has been added which should take care of it.
I'm just wondering why the pool would suddenly slip 'off center' for no apparent reason?

The only significant recent change was last weekend when I added 8 oz of HTH stabilizer.
Would that do it?