Snoke and Kylo had sensed her awakening before, but she became aware of her own abilities when Kylo Ren tried to pull the map from her mind during their interrogation. Kylo struggled to read her but she just shrugged it off, looked at him weird and turned the tables reading HIS mind and learning about his insecurities and even found out about Darth Vader from HIS memories.
I thought they played that out really well in the movie without needing any internal dialogue, but a lot of folks on reddit don't see it that way. Anyways, Kylo Ren basically activated her by accident.