I got the impression there was not enough of them in this movie to form the ball. There was way more in the 2nd one as the besieged an entire town.

It's foreshadowed earlier in the 2nd movie, four of them are seen to roll together while inside the burger joint and near the end of that sequence the large ball is shown to run into a post and split back into four separate Critters.

I never got the impression that this is something the critters can actively and deliberately do - more it was a result of them being 'blown' together in the explosion

I wrote that article, lmao. You only see it in the second film sadly. It was a pretty cool idea and I don't know why they never used it again.