keeping the challah braid even

Looks like you didn't get that final end sealed and that's where it came loose.  When i get to the ends I like to pinch real well and then tuck them under a bit allowing the weight of the loaf to hold the pinched ends down through proofing.

 Also looks like the dough could have been developed a bit further.  Since this is straight dough with a relatively quick bulk ferment and no folds it needs full gluten development.  I'd also shorten the bulk ferment to 1 maybe 1 1/2 hours as from the looks it may have been ready to shape sooner.  With a stronger dough shaped at the right time you'll have an easier time shaping and when it rises you'll get more of an upwards than outwards movement.  Or at least a better balance of the two.  I'll bet it tastes great though.  Hope this is helpful and good luck with your challah.

Josh