Grizabella the Glamour Cat - London Palladium 2015 

I don’t know about Rumpus Cat, but it is implied that Grizabella is Munkustrap and Tugger's mother. Possibly Macavity as well. It is confirmed that Munkustrap and Tugger are brothers so maybe Macavity is their brother as well. The Jellicle Cats hate Macavity because he is evil and will hurt or kill them. He's hurt Demeter before and it caused her trauma. For Grizabella, they hate her because it's implied that she betrayed them in some way. For example, leaving the tribe and if it is true that they're her kids, abandoning Munkustrap and Tugger.

its left up to the watcher's determination why, its not explained canonically at all.

@@c_jacket it really isn't that hard is it? There's a pretty agreed upon interpretation of the story people just tend to not understand/see (same reason the critics thought it was a bad musical at first, 'cause it seemingly had no story, but it definitively does). She isn't even seen as a threat, just as not welcome, given she left the tribe and happened to come back on this night. The tribe thinks she just came back for the ball and the potential to be chosen, even though she just wants to be back in the tribe, back to her origins.