shouldn't do this without it being told to - there can be a number of reasons - firstly, let us know what setup you have, and how you have each tuner set up in your tuner configuration, also do you have any other receiver that is controlling the dish with a pass through or another feed to the same dish? Do you have the EPG set to grab Opentv data at certain intervals? - in the recording settings, do your recordings take priority, and if so do you have any recordings that could be causing this?
There are plenty that can be causing this - you need to narrow it down and look at the settings to see if any of these are causing it.
