Does anyone have any solid theories? I'm worried that this mystery will be built up so much that people are going to be disappointed by the eventual reveal. What could Michael have done to make him stop being her shadow, withdraw from her and Amanda? What would make him repeatedly rebuff Michael reaching out to him in the years after the incident? Does it have something to do with his half human side? To be honest, I am having trouble coming up anything sufficiently emotionally devastating to warrant Burnham's obvious guilt and regret. That said, we know he's held an ongoing grudge with his father for 18 years going by 'Journey to Babel'.
She probably told him he's too human to be Vulcan, and despite her 100% human status she's a better Vulcan than he'll ever be, and Sarek likes her better (or whatever the proper Vulcan euphemism for "like" is). It's the opposite approach from Kirk in "This Side of Paradise" and Trek 2009.
Why does it have to be any one thing? It could be a culmination of her being introduced in to his life, impacting his relationship with his dad, and mom, and on and on.
In my experience the smaller and stupider something is, the bigger it's blown out of proportion. Or it's a collection of little things and one if them was the final straw. Maybe it was something Burnham ruined for Spock that was so important to him. I'm leaning towards this because it's the most dramatic. "A bunch of little things" and "A big deal over nothing" don't seem very epic. I think Burnham stopped Spock from having some sort of once-in-a-lifetime epiphany when encountering -- or trying to encounter -- the Red Angels.
Burnham told Spock that she checked the records, and there's no indication Spock's "cousin" Selek actually exists. She thus accused him of being illogical and having imaginary friends. At this point in time, Spock assumes that Burnham actually killed Selek and is covering up her crime since he can't find out what happened to Selek either. This feud continues until TAS' "Yesteryear", at which point once Spock realizes the truth he is so embarassed by his assumptions that he refuses to ever speak of Burnham again.
Before Yesteryear: Spock doesn't speak of Burnham because he strongly suspects her of killing his cousin "Selek", even though he can't prove it. After Yesteryear: Spock is so embarrassed at falsely believing Burnham killed a cousin who was actually his time-traveling self and didn't even exist that he refused to mention Burnham ever again. Family and friends go along with this and humor Spock to the point Burnham isn't even invited to his resurrection. Amanda is fine with this as she's still mad at Burnham. So, I think that just about covers why Spock and family never mentioned her.
Burnham introduced Stonn to T'Pring, at which point T'Pring didn't even bother having anything to do with Spock. Burnham then took photos of T'Pring and Stonn on dates and posted them all over the galactic warpnet. Burnham also told T'Pring to unfriend Spock from Vulcanbook.
Stole his binky and threw his toy Katric Ark Playset with Mount Seleya Attachment and Vulcan Elder Action Figures(TM) in the garbage.
I'm sure it's a build up of things, but don't forget she did say they'd been close once, and that he'd been her shadow. That she'd been afraid of him getting hurt in her vicinity if the logic extremists targeted her again. So she did something unforgivable. Her dialogue with both Pike and Amanda implies a single event which truly broke their bond as foster siblings.
Burnham: Spock, Stonn's 10 times the Vulcan you'll ever be. T'Pring wants a real Vulcan, not a half-breed like you. Spock:
She convinced him that his beloved brother Sybok left the family because of deep disappointment in Spock, and she continuously taunted and tormented him about it, while simultaneously choking his pet sehlat with the an-woon and bashing his computer drawing console with the butt end of a lirpa. Kor
I was thinking we've already seen the reason; Michael tells Spock that no matter how hard he tries, he will never be Vulcan enough for Sarek's mission and never human enough for Amanda's love... two things that Michael could have easily said she had achieved (even if she didn't). I think, depending on the age, that definitely would have been enough to piss off Spock.
After reviewing this thread, it seems that what the audience imagines Burnham did to Spock will almost definitely be far more horrifying than whatever reason is revealed. Maybe it's better if the Discovery writers never reveal what happen and just leave it to the audience's imagination.