Actually according to Kristian thats not what happened. Bran warged into Willas but not into present Hodor. He should have come out of the dream first. His hearing Meera yell hold the door transferred into young Willas but not older Hodor. Hodor was hodoring of his own free will
http://www.nytimes.com/2016/05/25/arts/television/game-of-thrones-hodor-feels-your-pain.html?_r=0
Bran was only compelling Hodor long enough to ecourage him to get up, he never warged out of him nor left the greensight - the connection to Hodor through Wyllis was still there even though Bran wasn't assuming direct control from the tunnel onwards. Wyllis' eyes never went white until the point where Wyllis heard Meera's words echoing back in time through Bran. When he turned to Bran and made eye contact - Bran warged into Wyllis while still maintaining the connection to Hodor. Which linked him to his future self - who was living holding the door, hearing holding the door, and being mauled to death. It can be assumed that Wylis experienced everything Hodor was feeling up until Hodor died. Basically Wylis experiences his own adult death as a child.
Bran didn't feel the pain because he was in the greensight the whole time and never really assumed direct control of future Hodor because he established the connection through young Hodor.
It doesn't make any sense why simply hearing "hold the door" would cause such immense trauma.
I like my explanation better :x