I think the seeds of doubt had already been placed within Zechs, and this just furthered the budding sprouts of those seeds. I also think that Zechs wearing red (as opposed to Treize's blue) is a bit of Red Oni, Blue Oni symbolism that Zechs is going to eventually do you-know-what (no spoilers :D) . . . in addition to the red being obvious because Zechs is the Char clone.

Sorry for late reply; I'm terribly behind in my watching.