How come output LED turns red while volume is the same?

Hi,

I can'f figure out, but this must be a stupid question I think.
I've levelled out several rigs and the volume are about the same level. I did this by ear.
But somehow while in most rigs the output LED stays green, in some rigs it turns full red, the green is not even shining through anymore.
I know red means the output signal is to high but just as I said the volume levels are about the same.

So my question is, what exactly triggers the output section to make the LED turn red?

Thanks in advance.