I wouldn't take the dog back there. Maybe they tried to catch him and kicked him or stepped on his foot. Anyway the dog was most likely injured, hurt by the people there.
If this is totally out of the blue, and up to now he has been good with people and dogs.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC