One reason I've seen? Usually they're behind and trying to get to where they need to be. For example, a Master Yi who's behind a Lee Sin may stay behind if the Lee is able to keep himself viable in the late game. So sometimes they'll take a few minions or a few waves to try and get back in, especially if they're being counter jungled.
Though I've also seen it happen when the lane they're taking the cs from is behind. I've had a game before where I was 0/4 as Vel. I was having a hard game against a LeBlanc and so I was trying my best to farm under my tower, which wasn't going so well since Vel auto's don't do much and I didn't have a lot of ap. My jungler didn't help, but would take farm I was trying to get. The reason being "you're behind, i'm not. if i get more ahead i'll carry."
Personally, the only time a jungler should be taking cs is if;
- The lane is being pushed and the laner is either dead, had to recall or in another lane. ie; top/mid teleported bot to gank.
- The jungler is helping to try and get the tower, which then it doesn't matter WHO gets the cs
- The jungler is behind and is trying to get back ahead.
- The jungler just ganked, and YOU got the kill. If you got the kill, let them have some minions.
Though I'll admit, junglers should be more careful who they're taking cs from. A Nasus who's trying to stack needs the minions, while the adc will need them too if they're behind, ahead or trying to stay on the same "level" as the enemy adc. Most tanks will also need minions until they get 3 or 4 items. Often times it's hard to know who you should be taking cs from and who you shouldn't be during the entire game. Unless someone's so fed they have all their items, then they don't need cs imo.
But, you know, a lot of people also care about their end game grade and will complain even at full build if they don't get cs.