It happens, though Carson can't remember how or recall the why.
To the butler, Thomas is an aberration, a flaw in the smooth cloth that forms their society. The boy (And he is a boy, for no man would be so lacking in dignity) sneers at them all as they attempt a day's honest work, thin lips curling in disdain while his eyes flash as treacherously as ice. Thomas holds no reverence in his heart for the family, seeing them either as rungs on a ladder or obstacles covered over in shining gilt. He doesn't understand the power in Downton Abbey, doesn't realize the haven it offers. There are others out there in the wider world who bray like asses, their eyes dull and dumb as they claw at one another in squalor. Thomas has never been stumbling and stupid from sour beer, nor felt blood tricking into his eyes from a wound opened by a swinging bottle. He has never sacrificed the scraps of his pride for coins. He has lived and worked in the Abbey for years now, sheltered and offered dignity, a mantle of respectability.
It is enough to serve the family of an Earl, more than enough, yet Thomas keeps his eyes raised upwards, fixing on things people of their station are not meant to have.
It infuriates Carson, because Thomas could be great in service to the Crawleys; it is something the butler feels in his marrow. The boy loves the estate, loves the beauty and vastness of it all, and in Carson's mind's eye he sees him standing as mightily as a Centurion in her defense. Thomas could make something of himself, yet he sulks and snaps and paces the elegant halls as if caged, and Carson is steadily losing the last of his patience. In the beginning he tried to go on his intuition, to guide Thomas, but after years of pilfered wine and scornful barbs Carson has the footman crushed up against the wall of his sitting room, the blood pounding in his ears, Thomas' sharp jaw trapped in his hand.
"Why can you not accept?"
And thin lips curve into a sneer, defiance on top of half-formed fear.
"I'm no lapdog."
Carson does not remember twisting black hair in his fist, nor does he remember moving across that final fatal distance, but Thomas' teeth are scraping at his bottom lip and distantly Carson hears buttons go clattering off into shadowed corners.
Much later, when numbness has replaced panic, Carson sits amid the wreckage and thinks, perhaps, now he may finally be able to impress the proper attitude upon Thomas. Perhaps.
In the room at the end of the hall Thomas sits and thinks as well. He thinks of how far he can go now, with this little incident hanging over old Carson's head. He thinks of the rewards reaped, of coming and going as he pleases. He thinks of what solemn sacred rule he might shatter first, walking free of consequences.
It's not an ideal arrangement, sure, but it will serve. He's not ashamed of having mussed his hair or of having put his hand down the butler's trousers- if an artless, muffled fuck against the wall will get him what he wants (and he wants, endlessly and intensely), then so be it. Those painted, waif-like girls in York walked the streets and looked brazenly into the eyes of banker and butcher and butler alike, and what's good enough for them is good enough for Thomas (contrary to whatever Bates might think, Thomas is very much a believer in equality between the sexes).
And although Carson's older than Christ and terminally clenched tight like the muscles that shape a frown, he's warm and he's solid and very much here. He's got hands made clever by work and a voice that rumbles. He's someone.
So it's happened, and so long as it helps him and keeps the emptiness out Thomas doesn't give a damn for the how or the why.