We will never know the full details because a lot of the information would be private commercial information.
But to me the fact that it was only a single user who was issued the notice, makes it worse, because it would indicate a single user was capable of causing visible contention on that particular exchange which in itself suggests the shared capacity was small, I remember reading on TBB that BE were commonly using 100mbit pipes in exchanges whilst their competitors like SKY and talktalk were using gigabit in the same exchanges. If this was true, no idea but I remember reading it.
Also perhaps when looking at the bigger picture, if a single user downloading 1TB was a problem, then how would they manage FTTC? as 1TB per month on a 80/20 connection isnt that extreme, it would perhaps explain why we never seen FTTC services on BE. On a 16mbit connection (quite high but not unrealistically high sync for adsl2+) if pushed 24/7 the line would be capable of downloading about 5TB so such a line would be about 20% utilised over the course of the month. If you selling an unlimited service and one guy using his line at say 20% capacity makes things fall over, then that is perhaps poor planning.
It doesnt add up tho, if we assume a 100mbit shared pipe at the exchange which was the problem, and we assume the 1TB was all around peak hours as well so worst time possible for BE, and we also assume a high sync speed of around 16mbit, that is still about 84mbit of unutilised bandwidth which suggests there was a lot of load from other users as well and just that one user just was an easy target to try and get a quick reduction of utilisation on the backhaul and wouldnt have resolved the congestion but just made it less serious. So my speculation is that this particular exchange was one with good sales for O2, and they got caught out by the upsurge in users so no upgrade was carried out, and with an upcoming sale to sky, there was very likely an investment freeze so no approved upgrades and as such the only resolution available to network staff to ease the congestion was to try and lower the load from existing users hence the FUP notice been sent out.
As to the close proximity of the march handover to sky and the FUP been issued, consider that BE network staff may not have been aware of the march date (communication issues) and the time for meetings etc. to approve the FUP action so basically lag, then it can explain why the notice was sent so close to the sky takeover.