Well, from the trolls point of view, it's fun, and funny to watch people rage and to overall ruin other people's gameplay. Some of them do it just because they can, others do it because they have the mentality of "If I can't get <insert rank> because of trolls, I'm going to troll!"
Verbally abusing/making the game non-enjoyable for fellow players isn't just restricted to trolls. For instance, I just had a game this morning where I was first timing Morgana because of getting her victorious skin. I didn't know she did ridiculous amounts of damage, even on her base (300 damage Q on base, and then that scaling made me lol as I was 1 shotting 1/2 the enemy ADC's hp literally). So I of course was throwing out Q's and unintentionally getting every single kill. However, at the same time, their Blitzcrank was doing the same thing, leaving both ADC's to farm and be fed off assist gold. Their Quinn was fine with this.. Our Varus, was not.. He ended up screaming across chat, calling the jungler a noob for never ganking, and calling me a noob for taking the kills, and in the end, finally AFKed until Ahri and I made a play for Ahri to get a triple kill (leaving me to believe our Varus was just bad at securing kills, he was using Q in AA range without any W stacks on the target), where he finally turned around and said "I'm feeding, will be at dragon pit" in all chat, and that's exactly what he did...
In short, people do it because they either find it hilarious to do, and get actual enjoyment from seeing people rage, or they do it because the game isn't going exactly how they think it should go, even if you're winning.