Why would he act this way?

I am a 57 year old male and I was with family one day out on the patio just sitting around relaxing after a meal. I work out at a gym and just to make conversation I asked a family member if he still goes to the gym but before he could answer my nieces husband who is 15 years younger than me says with an angry voice "yeah thats what we wanta hear is how you workout and I can see someone asking me that. No one said anything for a few seconds because it was so weird. I just sat there looking at him as he looked around at the ground not making eye contact. I have never done anything to this guy and have no idea where that came from.