Phoenix Police Department attacks anti-Trump protesters

Does the City of Phoenix Police Department have a credible explanation for what threat their officers were responding to in the .019 seconds which passed between these two photos being taken?

These were shot by American Babylon photographer Jimi Giannattiduring the Donald J. Trump rally at the Phoenix Convention Center on August 22nd, 2017.

Photos Copyright 2017 Jimi Giannatti

Here are the full resolution original files for anyone interested in verifying that they are what we say they are: 
https://americanbabylon.prodibi.com/a/4xe29qqw6m81d6?st=4AvHG46WXk5YRlgUr%2Brn%2BF%2FfsY48kC2muljqRaousHU%3D