Aviation Herald's take on this accident:
Accident: Aeromexico B738 at Los Angeles on May 20th 2017, ran service truck over
A commenter claims to have worked for the company that operated the truck, and says "... from my experience, the trucks do have seats with seat belts but nobody used them. Also, most of the drivers drove recklessly and were undertrained to be driving on active taxiways. I am not surprised to see this happen."