when did Riven get so strong?

Really what makes her a menace early is her mobility with utility. I could sit here and list her advantages, but her big disadvantage is long fights early on. She heavily relies on her skills and if she uses them all, most Rivens back off and give respect until their skills are back up again. Know her cooldowns early on and you should do okay against her.

They didn't - just a lot of champs got nerfed into oblivion and finally she is back! With tons of dashes, AD scaling shield, execute damage, super low cooldowns later in the game and the usual early game cheese potential ;)

Champions have less health until maybe level 13 or so thanks to the new stat gain system. Riven already had pretty massive burst so it's just that much easier for her now.