It's not really unheard of, a lot of people don't know how to play a 5 versus <5 if the enemies are ahead. For whatever reason players often think they need to push and win fast when the surest way to lose such a game is to risk further snowballing the enemy heroes that are already bloated with increased xp/gold gain and leaver gold. I've always been of the opinion that if you're playing 5v4 or something and you really want to be sure you will win (or if the enemy is a lot ahead of you) the best way to play is to have everyone farm, fight only to defend towers and play as safe as possible. No matter how fed the enemy carries are, they will max out at level 25 and 6/6, but if your team has enough items then your higher numbers will win you the game. If you throw your team of 1 carry with couple of items and 4 supports with nothing but boots to a level 25 40.000 gold net worth solo carry, you're really deserving to lose the game. If you sit back and get farm on your carry, a hex or two and at least a Ghost Scep on every support you'll easily win.
Which is basically why I really, really hate playing anything but 5v5 and usually leave a game after an abandon. It should be common courtesy for people to just leave if someone on their team abandons early on but for whatever reason win-loss stats seem to be really important to some people, and I've had to endure some rather boring games because people on the other team didn't have the courtesy to leave and I'm queued with some friends who don't want to take a loss. Few days ago I was on my alt account and queued with 2 friends and a someone in our team quit like 30 seconds into game. We also left (even if 3 enemies had also quit) and I stayed in the game just long enough to see the last guy in our team get really angry in chat and demand that we stay. Later on I looked at my game history and noticed that game turned up as a win because apparently that last guy had played 1v2 for 40 minutes just to get a nice win in his record. I felt pretty bad for those 2 other guys.
Case in point, this game:
https://dotabuff.com/matches/45252715
I've been hoping to forget that game, but anyway. 3 enemies quit some 15 minutes into the game at which point their Tinker and Void were 5+ levels ahead of everyone in the game and both essentially had free farm paired with some kills. I wanted to leave but was asked not to. Considering how big both their heroes were I suggested we farm some items before we do anything (WR, Axe and Lich had almost nothing but boots) but they insisted on trying to push right away and repeatedly got disintegrated by the Void/Tinker combo. In the end, it was my fault for thinking it was not a good idea to go pushing with a level 6 Lich with boots and branches against a level 14 Tinker with BoT, Blink and Dagon.