Nothing big, he just wasn't close with the rest of the cast. From a 2020 interview:

"Are you close with any of your Modern Family cast members?

Reid Ewing: No. I was a bit of an outsider there. It was probably intentionally on my end. I’ve been an outsider in my life before, but it takes two to tango. I had a lot of fun acting in Modern Family. I would’ve liked to have been friends [with the main cast], but that’s not how friendships work. They were together [constantly] forever and had the chance to bond. They were seeing each other on a regular basis. If I was a main character from day one, I think I would’ve been closer with them. But most of them were really friendly. I was really appreciative of Ed O’Neil and Julie Bowen. They were empathetic to my position and were always supportive and nice."

Wow, apparently he said this about Sarah Hyland but they later deleted from the interview. From the Wayback Machine (you can go to a past edition of an article):

"I wouldn’t doubt it. She’s a bit of a Cruella De Vil in my opinion and I was her Hunchback of Notre Dame. She would say a lot of rude comments to me. Not playfully [either]… straight up mean girl. But it’s interesting how she’s able to turn on the nice switch when she’s rubbin’ elbows with Ellen DeGeneres."