As has been explained above. It is in the NYT article, the "truck" ie tractor unit came from Ireland, and the trailer unit was registered in Bulgaria in 2017 and its journey thereafter could not be ascertained by Bulgaria.

The tractor driver was presumably summoned as an emergency measure. And the delay, is easy to imagine, resulted in the deaths.

And as for hanging this on Brexshit, it is bound to be so hung. Brexshit is an unholy mess and in an unholy mess there are many hanging threads (to hang on). But also, during an unholy mess the smugglers probably saw an opportunity to hide their movements.

Anyone remember Alistair Campbell's e-mail wot sed "this could be a good day to hide bad news"........ the smugglers must have thought along the same lines. Their original plan clearly didn't work.