This has indeed happened got me. Both mrs t and myself got DC offered on a trip with an 8 hour connection when there were several shorter connections available (it was to LHR). In fact our connection was the LONGEST on offer (and we selected it because it was the only one where a J Saver was available). We were on 2 different tickets (because she was on a different inbound flight), she on J Flex and I on J Saver. So completely independent. And we both got offered DC. No idea why, but we gladly took it (although I must say that 8 hours is about the minimum time for which a DC is attractive IMHO).