The mountain crushed that the targaryen baby's skull because the Targ was going to grow up to be the next Hitler. The Mountain was a time traveling Andre The Giant this whole time. Ta-daaaaaaaaaaaa!
This is nonsense tbh, everyone knows Sean Bean only signed up as it was for a single season role so he could then concentrate on his main life passion*
Really interested to see what the mountain will actually look like. They're going to need some makeup to make him not look like a 24 year old behemoth.