Why'd they do this?

I was in a ranked SR last night, and we were losing badly. After 30ish min of gameplay, the opposing team was smashing our nexus, and they ff. I have had this happen a few times in ranked TT and once before in ranked SR.

Why'd they do this? I assume they were a team of smurfs and just wanted to destroy for a bit then, out of the kindness of their hearts, give us a win. Is this a thing?