There could be a number of different reasons that an event may have been cancelled from your diary without you manually carrying this out yourself.
Estimates:
When you have marked the estimate as accepted or rejected by the customer, any diary events in the future will be automatically cancelled.
Jobs:
All diary events related to jobs in the future get cancelled in the following instances:
You have raised a final invoice.
You have aborted the job.
You have marked the job as ' Free of charge'.
Commusoft does this because if the events do not get cancelled then the engineer may turn up to the property unnecessarily.