This has happened a lot today for me and seemingly at random. I can only deduce that something happens when more than one action is trying to be done and it doesn't know how terminate the previous action cleanly.
This happened while my primary Sim was trying do a woodwork project and was stung by the nearby bees.
It happened again when he was writing a computer program (just sat there with a blank stare)
This happened to my Sims date while they were out on a lot. The entire lot ended up resetting shortly after this too (drinks/books dropped, sims snapped to a default pose and remingled)
This happened to my Sims cat when it ran away and came back, the welcome back just wouldn't work while it was stuck in place on a bed.
I do not have a low end PC too, it far out weighes the required specs for the game and no frame rate drop occurs at any time. The sim just does not respond to inputs for up to 5 mins.