I don't remember every single CS:GO match I've ever gambled on, but one that was a big upset and was a massive win for me was NiP vs Vega Squadron at the first ELEAGUE Major main qualifier in 2016. Vega Squadron ended up winning almost 16-2 again NiP, and HLTV even shows you the scale of this upset
(click for match link). NiP was considered one of the top teams in the world at the time, and Vega was still on the come up at the time. I didn't bet an extremely large amount on Vega, but it was funny and surreal at the same time that NiP was choking so hard.
I always feel that in CS:GO anything can happen maybe it is because the level is close for all players or maybe there is some matching fixing or anything involved but betting on favorite team is a big NO for me because I am quite sure that the underdog team will at least win 1 map in 8 out of 10 matches and getting crazy odds is normal.
I still wonder how sometimes Navi looses to small teams but beat Astralis and other top teams. Always has been a dilemma and I feel that maybe it is because big teams prepare big for bigger matches while being reckless against the underdog teams and that might be the only reason behind such chokes. Another thing that matters a lot is teamwork because we have seen how an average team can a menace if they are well coordinated.
Betting on an underdog is sometimes I consider a crazy move unless they have the history that can possibly defeat the mainstream teams. Getting crazy odds is one of the other reasons why someone is betting on an underdog. Upset happen greatly on esports games and we can't really be sure what team will win on a game.