I and my colleagues travel on BA MAN-AMS very regularly, and flight cancellations are a regular feature. On the plus side they seem to happen a lot less now that the flights are all E145s rather than 3 737 roundtrips/2 E145 roundtrips; on the minus side they do seem more prone to cancellation during quiet periods (which late August definitely is as far as business travel is concerned). Thus we have always assumed that passenger numbers were the reason.
8 years or so ago the flights used to be pretty full most of the time, but as KLM ramped up the number of their flights, and Easyjet started out of Liverpool the BA seems to have been hammered. They also lost a lot of passengers from the first MAN departure of the morning as that flight got a poor reputation for ATC delays - I usually spent more time sat on board at Manchester waiting for a slot to Amsterdam than I did airborne. However recent experience suggests that moving the departure time to 20 minutes earlier has helped a lot. It also didn't help that when sitting waiting the KLM FK100 usually taxied past - although they also got hit with ATC delays their departure was 0610 but the perception remained.