He and his family were allegedly threatened in the summer after they won the CL in 2005, when he was wanting to leave for Mourinho's Chelsea. I remember it at the time and it's pretty much an open secret. He has very strong links to Liverpool's (and other places in UK and Ireland) underworld as Don says. so wouldn't surprise me if he's always been into all sorts of dodginess.
I guess another reason to add to why Gerrard was much more threatened than Trent (in addition to the ones Don's wrote) is that Gerrard was coming off all the hype of the CL Final, so he will have been seen as a messiah in Liverpool, much more than Trent is now, and they could never have him leave. Plus there's the fact that Liverpool are a stronger team now than they were in 2005 and can get by anyway.
The point is that Gerrard isn't the loyal Liverpool lad he is made out to be - far from it - and it's one of the myths of Gerrard's career that he is.