That dog on the soccer field was an accident waiting to happen.
We rescued a dog that laid down/rolled over on his back like that when you'd try to do anything with him. Every time you wanted him to do anything, he'd do that, then he'd nip and bite when you'd try to get him back up.
It's kind of passive/aggressive (if dogs can be such) because he'd act passive/submissive but if you reached down there he'd bite.
That dog on the soccer field reminded me just of that and I thought I saw nipping/biting behavior at one point although they finally got him up and away, but not by his collar, only by picking him bodily up and toting him off.