If it's just a signal indicator, it may be you are just getting a little crosstalk. That is, isolation between channels is not perfect. If you play loudly, you'll get more cross channel interference and the amp thinks you have something on the center channel.
Not sure how you have your channels arranged, but try moving the L and R to the far sides of the amp, if they are next to the C.
Not sure how you have your channels arranged, but try moving the L and R to the far sides of the amp, if they are next to the C.