Rance Preston (Earth-616)

I've been rewatching everything MCU and I'm currently on Loki S02.

I don't remember them exploring how Loki got to time slip.

So, I'm wondering, what caused Loki to time slip?

I understand that the "pulling him out of the timestream (all time)" via pruning would have helped him be durable against the radiation from the timestream and that's how he manages to enchant all time branches (taught to him by Sylvie in S01), making him "God of Stories" or whatever. But...

What caused Loki to time slip? What happened to him? What triggered that?