This is a failsafe mechanism.
Inputs via the ‘rc’ command need to be sent repeatedly. If the interval between received messages is too great, the input ‘times out’ and the ROV disarms. This is important in the case that something goes wrong with communication or pilot input, like if a joystick gets unplugged, or communication is lost from a crashed application or broken wires/connections.
Update your software if you haven’t yet, and then look at FS_PILOT_INPUT and FS_PILOT_TIMEOUT parameters, which control the behavior of this mechanism.
-Jacob