He could have been stressed while you were there next time he does it place a towel or a sheet or something over his enclosure to make it dark and make him feel secure leave him alone for about 30 mins and check back he should have at least started if not already finished eating it, worked for me a few times

another thing is it could just be cuz he is ready to shed soon, when snakes are in the "blue" stage of sheddin they cant see and probably sensed the mouse but was more scared and just wanted to kill it so it wasnt botherin him

i feed my snakes on a regular schedule:
yearlings/babies - offer food every 5-7 days
1 year and older - offer food every 7-10 days
if they eat they eat if not i will offer them food the next scheduled feed