DIAGNOSIS: That's the TV's Auto Picture or Auto Device Change feature being overly aggressive. It's designed to switch to an input when it detects an active signal, but sometimes it gets confused by cable boxes, game consoles in standby, or other devices sending intermittent signals.
FIX: This is an easy one to shut off. 1) Press the Input button on your remote. 2) On the input selection screen that pops up, press the Down arrow on your remote's directional pad to highlight the gear icon (Settings) at the bottom. 3) Select it. 4) You'll see an option called Auto Device Change. Toggle that OFF. This stops the TV from automatically jumping to an input just because it sees a signal.
IF THAT DIDN'T WORK: There's a second culprit. Go to Settings > Channels & Inputs > External inputs > Auto picture. Toggle this OFF as well. This feature tries to guess the best picture mode for the content, and its detection routine can sometimes trigger an input change.
HEADS UP: If you have a cable/satellite box connected, make sure its 'Power Saving' or 'Auto Standby' mode is turned OFF. These boxes often send a tiny wake-up signal periodically, which your TV can misinterpret as a command to switch over.