Interesting that CAI is also expanding.
FRA is #2 and CAI is #9 on slot constrained (or was before a BK at CAI).
So as I suspected:
It seems that if the primary driver for the airport expanding is lack of slots, and if the expansion does not (yet) deliver any slots, it seems that in more advanced game worlds, some airports will get back to back expansions.
since the infrastructure expansion does not grant new slots, the slot constrained airports will keep coming up first for infrastructure expansion...
PS: of the top 10 slot constrained, LIN, BCN, LGW are also expanding....