OK this was posted by 'Duncan' to give him credit... as it's not clear from the post the Doctor mentioned the Trickster in reference to the amazingly crap- time eating bug
Quote "In the Sarah Jane Adventures, The Trickster said he deflected all the events/attacks etc. that Sarah would have been involved with as part of his retroactive changes to the timeline to allow Andrea to replace Sarah. Since he seems to have been part of what happened to Donna, presumably he deflected the Daleks, Pyrovillians, Carrionites etc. in order to mess specifically with Donna's life post the point where she would have met the Doctor.
This wasn't explained directly in 'Turn Left', but then there wasn't really a point where it could have been brought up.
Either that or the divergent reality with the dead Doctor and the changed Donna ran in parallel with the one where the Doctor didn't die - and that Doctor went and dealt with all of those things (including with his Donna) up until the turn right moment when his actions no longer impacted on the parallel Donna in the alternate timeline. That would have continued, in his timeline, up until Donna vanished in Shan Shen, her history altered, banishing her to the alternate reality. Or the version of reality with the living Doctor and Donna would simply have ceased to exist - with impact on the past but not on the future, their branch of reality giving way to the new one." End Quote
Did that help, or make it worse lol I haven't seen the episode from SJA so can't agree or disagree with it really.