Not seen this reason before. Where is the congestion; Reading, Guilford, Redhill or Gatwick?

 
From what I can work out from RealTimeTrains, a Didcot - Paddington train was terminated in Platform 15. IIRC that train to Gatwick usually comes off the depot into Platform 15, so had to navigate other trains when it was altered into Platform 14 (at Reading).

 
I find that 'congestion' is announced when there's any sort of wait because of a conflicting move. (I refer here to crew announcements more than a coded delay reason shown somwhere like Journeycheck.)

I suspect passengers take it to mean several trains being in the way in the way that the term may be understood in the context of road traffic congestion, who may then wonder why 'congestion' was given as a reason when there didn't appear to be a lot ot other trains around! For example I've been on trains that were held outside the down platforms at CDF, so that an up departure could weave across, perhaps waiting a few minutes for that departure to take place. The TM announced on departure that the late arrival was due to congestion, but from the perspective of boarding passengers, the platforms were mostly empty, which may look like a contradiction! But of course 'congestion' is simpler than a detailed narrative about a conflicting move. Neither explanation will 'win' with everyone.

 

Agreed, and that's probably enough for most passengers ... "too many trains around" ... I might personally like to see some more detail, I suppose, but that can typically be gleaned from Open Train Times; more aware users can spot single lines blocked by out-of-path freights, platform shortages, etc