This seems to happen to me all of the time nowadays and I have no idea why. I will eventually go back and finish them sometime I just have too many games to play cause of Plus on top of buying more games that I want cause they're like 5-10 bucks and it's kind of hard to pass up at that price.
Red Sox World Series Champions 2004 & 2007= My ID. PSN ID- rswsc0407
Only on games that I know I can't enjoy any further. Examples include: Valkyria Chronicles where the first 7 chapters bored me, Dishonored/Deus Ex where I got tired of reloading check points for non-lethal runs and KH 1.5 Remix where I just hit Atlantica. Otherwise I try to make it a point to complete a game I like and unlock all the artwork/bonus commentaries. I won't bother with trophies/achievements.
If there is something really interesting that just hit my hard disk then that just got listed on my backlog...and I'll clear it.