There are several synonyms for the phrase "behind one," including "trailing," "following," "in the wake of," "pursuing," and "chasing." These words all imply a sense of being behind someone or something, either physically or metaphorically. "Trailing" often refers to being physically behind someone, while "following" can also refer to tracking someone's actions or ideas. "In the wake of" suggests that someone is following in the aftermath of something that has already happened. "Pursuing" and "chasing" both convey a sense of actively trying to catch up to someone or something that is ahead.