I think the driver of the supermarket delivery van failed to realise it was a railway crossing (I'm assuming the gates were left open as it's impossible to mistake otherwise) and proceeded without seeing the train or realising they were about to go over train track. Either that or they thought because the gates were open it was safe to proceed and didn't see the approaching train.
Thankfully we probably won't need any such speculation because Asda delivery vans have forward-facing dashcams as standard, so unless the camera wasn't working, it should be easily visible what the situation was on approach.Before coming to a conclusion like that, I would want to know that the delivery driver had never made a delivery to that site before. One of the crossings that I was responsible suffered from frequent misuse - with delivery and other 'professional' drivers being the most frequent culprits. Eventually, after several very near misses, the crossing was upgraded to be a AOCL. It was after that when one of these drivers got caught out by a train that he could not outrun. That driver had used the crossing almost daily...