BA and AA consider themselves one airline across the north Atlantic, so there are 3-4 other daily LHR-DFW flights to move people onto. And plentiful other connection opportunities - bear in mind nearly everyone on the BA flight will be connecting and most stations are also served from one or more of CLT, MIA, JFK, PHL, ORD, PHX.

BA are suffering with longhaul aircraft serviceability at the moment and as such a few flights a day are being cancelled. I imagine they’re picking the least-worst each time, which explains why the cancellations seem to rotate between ORD, JFK, EWR, BOS and DFW. And not every A380 every day is full.