Could someone please help me find out the reasons for a flight cancellation and then a subsequent delay on the rebooked flight.

Originally booked on the BA0357 on the 12th June which was cancelled on the 11th June, both BA on the phone and checkin staff didn't know the reason for the cancellation.

Was rebooked on the BA0349 on the 12th June (leaving 1hr 40 mins earlier than original flight), which was then delayed by 2hr 47 mins, the pilot just said it was due to Heathrow disruption.
Would both of these flights be eligible for EU 261 compensation, or just one?

Thanks in advance