There are several reasons for this.
1. The source device (ie., STB/Cable box) is being turned off.
2. The source device (ie., STB/Cable provider) has sent an update to the source device which causes the source to reboot.
Turning the source device off or rebooting breaks the HDCP network protocol and does not properly re-initialize the HDCP connection so the MWTV Transmitter needs to be power cycled.
We recommend you turn off the Tv and not the STB/Cable box.
If you continue to experience this issue then verify the settings of your source device as listed in this article. What-settings-are-required-on-the-source-device?