I'm not certain. I had a couple of theories - I had just moved the tomatoes back outside before the attack, but the tomatoes were next to a bunch of houseplants inside and none of them have spider mites, so I don't think it was the tomatoes.
I had put the dead flower ends of some carnations in the soil added to the mulch, which is a possibility - perhaps they were in the carnations.
I had also added a layer of compost to the top from a bag I bought at the store, but I also used that compost on a bunch of other plants and haven't seen spider mites in them.
So my best guess is the carnations - so I won't add flowers to the mulch anymore!
It could also be the heat, but this is her third summer and she was much smaller before, so my thought process was more about where the spider mites came from rather than if she was susceptible to them, though since I realized she had them, I've been deep watering her more and giving her extra food - more water was recommended by an arborist as a tactic to use on top of treatment. So the susceptible part might have been heat stress/needing more water in said heat.