Just received an email from Southwest informing me my flight to Nassau is cancelled.
Also today, my wife got an email from Southwest informing her flight to Park City is cancelled.

But the cancelled flights are all bookable on Southwest website!

Based on the replies below - it seems that the double bookings is the reason for the cancellations. I have never known this is the rule and am glad to learn something new.
Possible reason -
Bahamas flight - I booked two flights on the same date to Nassau, one via BWI and the other FLL. Southwest may consider it a double booking, so IT cancelled the one via FLL. If this is the case, how would they know which one to cancel.
SLC (Park City) flight - wife booked another on the same date for Denver.