info. "The teenager had been an werewolf the whole time. In face he was a born wolf and has been an Alpha since his mum past away a few years ago. Stiles hid his scent behind herbs, making him smell human to every other werewolf. Now it all made sense. Why Derek’s wolf always was so on edge near Stiles and why he remembered Stiles mum being at his house when he was little even though she never smelled like werewolf or pack. It also explained why Stiles was so good at dealing with Scott when he first got turned and why Scott never had to join Derek after Peter was killed, Stiles was his Alpha even when he didn’t know it. But it opened up new questions like why didn’t Stiles tell them? Or at least tell Scott. And why did he hide behind the facade of the fragile human for so long? And why did his mum do the same?"
In short, Stiles is a born werewolf. His mother was the Alpha of a very small pack until her passing (which is still fuzzy on the cause) and Stiles then became the Alpha.
With the Alpha Pack around and causing trouble, he takes it as a good time to tell his secret since their numbers are scarce. Even though it throws everyone for a loop.