My guess is their TCS designed ESB failed. AFAIK BA don't use native Amadeus Altea FM for Departure Control functions.
They have their own bespoke front end built onto the standard Amadeus product.
So other airlines weren't affected.
Quite what they are doing making (reported elsewhere) config changes during the summer peak is anyone's guess.
Everyone I know at BA (good people, VERY good people) has left/been sold off so I can't get any actual information.
Which says a lot about IT at BA.