[Caged Great]

I think Wyllis was linked mentally with his future self and "Hodor" was telling himself that he needed to hold the door. As his future self was dying, Wyllis' saying hold the door was getting slurred more and more until the moment of his death and "Hodor" was born. That was the point the connection was severed and it froze his younger self saying Hodor from that point on.

Wyllis was "struggling" looking as if he was pushing back against the ground during his seizure, but as Hodor was dying at the hands of the wights, his seizure was calming down as the life force of Hodor faded.