Former U.S. veterans were shocked at how listless and unprofessional Trump's parade was, and the theory is that American soldiers were annoyed at having to perform for Trump's birthday, and didn't give enough of a shit to perform to a high professional standard.
View: https://m.youtube.com/watch?v=RmvUx6vcnjM
www.buzzfeed.com

Here's One Veteran's Take On Why Soldiers Were Marching Like That At The Military Parade
"Every soldier learns how to march during basic training, and it's not hard."