It was the motor failure that caused the crash, not the return to home switch. That could be better verified by reviewing the telemetry, but for now, it is the most logical to offer. Motor failure can come from many causes. We have almost no way of offering a better explanation without the telemetry files.
What was the altitude?
How long was the flight?
Did you receive any low battery voltage alarms?
What was battery voltage reading?
How old are the batteries?
Any possibility of hitting something?
Any possibility of a bird strike?
Have there been any other issues with the drone lately?
Were you using Yuneec original propellers?
What was the reason for activating Return to Home?