Yang has said before that Summer died when Ruby was young, and Ruby clearly remembers her some, but not clearly. I'd guess that Summer's death was probably when Ruby was 5-6, about a decade ago.

Leo lost track of the Spring Maiden a decade ago. She walked out of Haven and was gone. We know from the most recent episode that Spring was not Vernal, but Raven, who was using her as a decoy.

Ozpin would not have been happy that one of the Maidens had wandered off. He probably would have, at least initially, had someone attempt to follow her.

Enter Supermom.

We have little reason to think that Summer wasn't the same sort of idealist that Ruby is. She probably bought in to Ozpin's goals, and it's not far-fetched to think that she'd be involved in hunting down the wayward Spring. She might have even been able to convince Raven to help her locate the missing Maiden.

A maiden who didn't want to be found, and who, despite her inexperience, was almost a match for two trained huntresses who knew what they were facing. It wasn't Hazel who killed Summer. It was Spring.

The maiden mortally wounded Summer before she finally was felled by Raven's blade, and with only two women on her final thoughts, the power went straight to Raven. It could not have gone to Summer, because that would mean that it would have bounced to Ruby or Yang when she died.

Of course, Raven knew what a shit hand she'd just been dealt. The death of a teammate, of the wife of a man she most likely still cares for, and the sudden assumption of a power that makes you a target is more than enough to explain her bitter distrust of Ozpin. She knew she needed to hide, because revealing what she'd become would end in her death.

So Raven hid her face, she stopped visiting Tai (since she couldn't look at his face anymore)^1, and she got herself a decoy (who was most certainly in on the deception, knowing that Raven could end her with a thought if she tried to betray her). She rescued Yang once, but wouldn't do so again while she was still at Ozpin's side.

1. This part isn't even consequential to the potato.