United are paired with us, Dortmund are paired with Bayern, Barca with Real, AC with Juventus, Chelsea with Arsenal for TV and policing reasons:
Look at the draw:
Group A: Porto, Dynamo Kiev, Paris St Germain, Dinamo Zagreb
Group B: Arsenal, Schalke, Olympiakos, Montpellier
Group C: AC Milan, Zenit St Petersburg, Anderlecht, Malaga
Group D: Real Madrid, Manchester City, Ajax, Borussia Dortmund
Group E: Chelsea, Shakhtar Donetsk, Juventus, FC Nordsjaelland
Group F: Bayern Munich, Valencia, Lille, BATE Borisov
Group G: Barcelona, Benfica, Spartak Moscow, Celtic
Group H: Manchester United, Braga, Galatasaray, CFR Cluj
City/United, Arsenal/Chelsea, Barca/Real, Benfica/Porto, PSG/Lille, Juventus/AC, Dortmund/Bayern all on opposite sides of the draw.
Thus when it came to the last 3 teams(I believe), Nordsjaelland, Cluj and Dortmund, it was clear that Dortmund were to be drawn in the opposite half of the draw to Bayern so had to be drawn in the top half of the draw, the only remaining group was D, ours. Nordsjaelland came out first so were drawn into E, Cluj then had to go in H and Dortmund into D.
Way it works.