Being in the wrong place at the wrong time .
Going by the video, it looks like the catering truck was eastbound in the vehicle travel lane and crossed the taxiway before coming in contact with AeroMexico. Probably crossed well before the B737 entered the taxiway but had to stop for traffic ahead (probably giving way to a plane on a taxiway further east) not realizing they were encroaching on the taxiway behind.