I think it's the only conference whose fans chant their conference name at games. Like you could be at the big Vandy-Miss St game getting your butt kicked, and for whatever reason the fans just start chanting SEC, SEC, SEC. Weird.
I highly doubt you would hear an SEC chant at a game between Vandy and Mississippi State. That would be very weird.
The chant is usually performed during big games featuring a SEC team and a non conference opponent and it's a form of trolling because SEC fans know how much other schools hate it. Very few fans of SEC schools actually root for their conference brethren for any reason than beyond the fact that the SEC pools it's money for bowl season... so you obviously want your brethren to do well because it means your school gets a bigger paycheck as well. But realistically, most SEC fans don't give a hoot about their conference brethren.. the chant is just trolling.