Maybe it's a tribute to anons, kek
They have masks on so their faces are hidden and all wearing Trump shirts
Engineers were a part of the troops being deployed to the border
The Wall Street Journal reported that the deployment would be focused on Texas, Arizona and California, with mainly military police and engineers from the Army and Marine Corps comprising the force.
https://www.militarytimes.com/news/pentagon-congress/2018/10/29/white-house-plan-to-send-5000-troops-to-the-mexican-border-catches-pentagon-off-guard/