With Sage, it's just that she could suddenly go from (what seems to be) very calm to very aggressive. Maybe not necessarily physical, but certainly in her speech. One of the most perfect examples of this is when she confronts Sarah and Mel at the pool and it's also the reason why MC is/was scared to ask Sage about her personal life.
Also a tsundere is not supposed to get aggressive, that's the yandere. The meaning of tsundere is " Tsundere is a Japanese term used to describe a character, often in anime or manga, who initially displays a cold, hostile, or aloof demeanor, but gradually reveals a warmer, more affectionate side ". This definitely fits Sally in how she is with MC (also Bella btw).
Sage's temper is too common knowledge for her to be a yangiru. The character must have a solid image of being friendly and calm so their temper comes as a shock. Sage loses her temper too often to count. Jill is much closer, but her temper doesn't go off hard enough. The yangiru is somewhere in between. Typically as calm as Jill, but sometimes goes off like Sage.
Tsunderes get aggressive all the time. The term started as a blanket term for characters that go from tsuntsun (in a sense cold^2) to deredere (warm^2). These days it refers specifically to the aggressive type, while the other types have coined their own terms. Bella has her outbursts, but she is more of a kuudere.
A yandere on the other hand is sort of a tsundere in reverse. Someone who starts out bright, positive and loving, but then turns out to have a dark side and typically extreme enough to be called psychotic. The idea that love justifies everything is a very yandere one when taken to extremes. If for example it turned out that James actually came back a couple of years ago, but before he met with Bella he ran into Jill and she killed him, then that would make Jill a yandere.
Quinn's actions don't make her a yandere because she never pretends to be nice or think that love justifies those actions. She gets revenge for Riona, but there are no delusions about being a good person.
Tybalt has definite yandere tendencies, but it's debatable if his actions were extreme enough. Also the fact that so many see through his act might cancel out his yandere status, but since Jill falls for it (at least initially), he should still count.
Bella's attitude with the Tybalt and Jill situation has a yandere vibe, but Jill straight up asks her to help and her actions don't get extreme enough. She even feels shame over sending a tennis ball to his face.
