I may be wrong, but I was led to believe that all the preallocated seats for Exec Card holders (who had expressed a preference in their profile) were reserved before the flight was open for check-in online, this would presumably include the seat that you reserved when you booked. Your seat would not be changed just to seat a cardholder with a higher "status" card as they would be seated according to their preference after your prebooked seat had been taken into consideration.
All that I can think of is that there has been a configuration change and there are less seats available of the type that you reserved which, in effect, meant they had 2 people reserved in the same seat. I suppose that the higher status card would then take priority.
It is a shame that all the same types of aircraft in an airline cannot be in exactly the same configuration, I presume that this is a historical thing.
Finally, the fact that it has happened twice is probably an unfortunate coincidence.