It just means it passed the anticollision and the reader was able to read out the tag identifier and select the tag.

Presumably the reader would do exactly the same if you waved a London Underground oyster card at it.

Edit:

I misread that it was apple pay screeen coming up on the phone, same reason though.

The anticollision the method by which the reader is able to determine the identifiers of all the tags in range (go read the specs on these if you like, they're kind of interesting...really!), so it selected each tag it found in turn, obviously one of which was your access tag which opened the door, it also selected the apple pay tag which would appear to be enough to wake your phone and get it ready for some apple pay action, but then stopped as it wouldn't have known what to do.