Most of the stress fractures on the Model 3 rear glass have historically manifested as just a growing "line" or fracture limited to the innermost layer of the glass. There have been cases of people getting warranty claims rejected if an actual crack could be detected (with a finger, pen, etc.) on the surface, as those kind of cracks are usually associated with impact with an object.
I don't claim to know the physics behind how automotive glass cracks, so perhaps Tesla introduced a new manufacturing flaw where it can crack like this, or something actually hit your rear glass without you noticing. Several months ago, there was an active thread on TMC where a handful of us noticed the exact same kind of rear glass fracture manifest on our cars (size, shape, origin point, etc.), and it was too coincidental to be isolated incidents. Unfortunately, it was during a time of a nationwide backorder on the rear glass as well, so that was not fun.
Anyway, I hope Tesla makes it right for you, and there is not a long wait on the rear glass.