In some settings, such as the National Union of Students, the insult is so pervasive that anyone to the left of Hitler is liable to be labelled a "trot" at one point or another, because the NUS right-wing associate any challenge to their rule with a conspiracy allegedly initiated by a long-defunct Trotskyist group called Socialist Organiser.
The term also crops up sometimes in anarchist and ecological attacks on Trotskyists. It is also occasionally used by Trotskyists themselves, apparently as a "reclaimed" term, only heard within the Trotskyist in-group, although this is contentious and some reject the term completely.
Its derogatory implications probably come from its associations with the trots in its other sense.
The trots are trying to take over this campaign group