I haven't tested exactly when all of this happens, but there are two conditions that will trigger a Rewind:
1.) Seism's health is below a certain threshold that makes him go into defense mode, targeting your creatures from "biggest" threat to smallest. 99% of the time, creature's attack power is the factor in determining threat level. If two creatures are tied for the highest threat level, then the creature in the earliest slot will be targeted.
2.) If the creature's threat level is high enough, Seism will Rewind it before the health threshold. For this reason, if I have a lot of earth quanta for some reason in the early game, I might PU a shrieker to try and draw out a couple of rewinds if I can afford the loss of a card draw or two.