Why is "at" being used at the end of the sentence?
"At" is used at the end of the sentence here because the sentence only ends since he interrupts himself. The full sentence it is implied that he would say if he didn't interrupt himself would have been "sometimes I don't like using the bathroom at work". So this sentence wasn't supposed to end which is why it seems weird that "at" is at the end. He interrupts himself and ends the sentence early in this case because he has a change of thought which makes him change his mind about the sentence he is speaking.