Wrote this on an earlier thread - it's surprising yes, but also in a way no, because we have seen this so often over the course of history from college to pro meets.
Everyone backs their kick, they just do. It's not the rational, conscious part of the brain that controls it because, as people correctly point out, why would a guy as good as him ever leave it to this much chance. His subconscious mind simply assumes that as the best guy in the field that he will be able to deal with any type of race. Problem is when you are running that slow, even 4 flat guys can whirr the arms and legs in the last lap and by the time you realize it's happening you are in trouble and it's just too late.
Ultimately this will be a positive moment for him. His first big NCAA meet and such a great learning experience that will sit with him and will mean he won't make this type of error in judgement again. And it works out well to have this learning experience in the NCAA indoor meet which ultimately isn't that important compared to outdoors or an Olympic/WC heat/semi-final.
Sam is a pretty chilled guy, I don't think this will bother him too much. He will be back, come outdoors, no worries.