Agreed; the actual vehicle is irrelevant, what the OP described was poor passing behaviour. If the road ahead narrows, you have to check for people merging from the closed lane into yours, whether you are cycling or driving.

In addition, the lead riders in the pack had a responsibility to evaluate the road conditions for *everyone* following, not just themselves, and it sounds as though they did not do that. It would be similar if the same group approached a traffic light that was about to change. The lead riders would not ride through leaving the trailing riders to either run a red, or be cut off at the intersection. At least I hope they would not!