frighten those Haitian immigrants (see linked article and photos)...whether or not their reins were swung in a 'whipping' fashion doesn't excuse grabbing those immigrants by their shirts in what could have caused serious injury...THAT is what Mayorkas was upset about, and hopefully such behavior won't be repeated.
Link: https://www.npr.org/2021/09/21/1039230310/u-s-border-agents-haiti-migrants-horses-photographer-del-rio