Maybe center tank didnt get filled up enough. Or because it went higher and burned for more time than the two side boosters that detached earlier, it needed more for its descent.
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They use a special chemical to start the engines. For some reason there wasn’t enough to start two of three engines for the the last burn. SpaceX has been igniting these engines for a long time now without any problem, so my guess is some type of damage that caused it to leak.
I believe the SR71 could restart in flight with that chemical but only get 3 or 4 charges/tries