Well it can only be PF3 and it's basically down to either timing, the AFCAD in use, bad luck or all of them!
PF3 freezes ground AI traffic when you're on approach but only at a certain point in your flight... obviously it won't freeze any aircraft that is on the runway or has been given takeoff clearance, for obvious reasons. It will also slow down traffic behind you whilst on approach, subject to certain criteria being met, in an attempt to help with separation.
One of the problems of course is PF3 does not control Ai traffic... that is done by FS and so we don't actually calculate where all AI traffic are in relation to you of the active runway (although there are certain times we do re the runway) and we simply go by a flag setting, read from FSUIPC, as to the current mode of the aircraft. i.e. Taking off, landing, rolling out, taxiing, etc, etc. So as I say, it could simply be bad luck with timing issues where as if you had arrived at a certain position 1 second later or earlier the problem would not have arisen.
Sorry I can't be more precise and as you can imagine it would be very difficult to ascertain the cause of any one particular AI traffic related issue, although I'd be happy to try if you provide a log and as much detail as possible when any such problem arises again.
I believe, by and large, PF3's handling of AI traffic works well but it's not perfect. I've always tried to ensure something silly doesn't happen... like freezing an aircraft actually on the runway whilst you're on approach which, given a bit more time would probably have taken off before you actual land. But it's a tricky one to get right.
Having said all that I have been saying for some time that I intend to revisit AI traffic monitoring and interaction but I'm not making any promises as to when that might be.