Manchester, like most major airports, has Standard Instrument Departures (SIDs), Noise Preferred Routes, ATC Radar, and a
LOT of traffic.
SIDs are issued prior to departure, and for any given runway, there may be several different SIDs all ending in the same place but with different routing and altitude constraints in between. The SID given 'on the day' may not be the most efficient for the flight, but essential for ATC separation or Noise control. It's common in my experience for departures from MAN 24L to Europe and beyond, to get the SID with a right turn to the West / North-West before tracking East some time later. There's no reason why aircraft B following aircraft A to the same destination should have the same SID, in fact a lot of good reasons for traffic separation purposes to give A and B different SIDs.
Traffic is often the overriding factor, and for separation, ATC may have given a Radar vector to your flight, if in fact, as you stated, everyone else had gone in one direction, sounds like that area was becoming saturated and sending your flight somewhere else was a good idea.
The big thrust cut-back would more than likely be an altitude constraint requiring a large cut-back from the high thrust levels for Takeoff and initial climb to a much lower setting for low altitude, low speed operations - normal. Sounds alarming because of the large change.
Is this an ATC influenced decision or is it captains prefered departure route
ATC in Manchester is VERY VERY good, but alas, beyond their control, the captain's preference rarely has anything to do with it. The Captain's preferred routing for weather avoidance, on the other hand, is always well accommodated, that too, may have been a factor.
The point that I'm trying to make is that there's too many variables involved to predict with any certainty what the Departure / Arrival will be on any given day or situation. As it happens, I'm operating to MAN soon, I have a pretty fair idea of what STAR and SID I might get (ROSUN comes to mind), but stand ready for any number of changes.