Mediaite has the story:
Young had briefly left an off-the-record GOP meeting about the border crisis and attempted to re-enter through a side door, and according to House rules, members are not allowed to re-enter through that door while a meeting is in progress. The aide was stationed outside the door to prevent people from using it — a fact that Young was seemingly unaware of, as a Vine captured by NBC producer Frank Thorp depicts. In a statement given to Politico, Young explained that he “was caught off guard by an unidentified individual who was physically blocking [him] from reentering the room. Regardless, my reaction was wrong and I should have never placed my hands on the young man.”
Now in his 22nd term, Young is the most senior Republican member of the House. So he should know the rules.