trolls & fudsters do what they do because of a veriety of reasons. some are crazy, some are smart, some are stupid, some have a hidden agenda, some are just bored, some work alone and some are part of a group. every individual is different.
To add up...
Some are jealous
Not to mention - believe it or not - some are just plain stubborn. They size up a worthy altcoin as a "scam," get shown up, and then spit out the humble pie that others would gamely eat. Consequently, they never tire of badmouthing the alt...