Why the hell is this funny?!

Me(18) and my boyfriend (19) were shopping for the house. We are staying with my parents for winter break until we go back to college. He walks with a walking stick/cane due to injuries from abuse/childhood trauma. Some days he is better than others. As we are walking I see someone run past him and he falls. Some girl ran and grabbed his cane and he fell. She ran to a group of other people who recorded the whole thing and some yelled "It's a prank bro! Don't cry. Don't cry! It's a prank!" While laughing at us. I helped my boyfriend up and he was in so much pain that he started crying and they laughed and said "Sorry bro" and left his cane a good several feet away. I helped him to the car and finished the shopping and walked by the same group and they were laughing and I could hear them talking about my boyfriend and him crying like it was funny. I finished the shopping and now my boyfriend is hurting more. I'm not sure what the punch line of the joke was. How is this funny? I don't think hurting people is funny. Plus they looked around our age. I thought only middle schoolers did this type of stuff.

Edit: I asked my boyfriend if he wanted me to talk to security or something but he just told me to get the food and let's go because he was hurting so bad.