What this conversation with Karolina could indicate instead, is that Shiv made her decision before the board meeting even began. Even though she was clearly angry that Matsson chose Tom (Matthew Macfadyen) over her for the US CEO position when she first found out, she clearly felt like aligning herself with him over Kendall would be better in the long run. After the deal goes through, Tom asks for Karolina specifically, indicating that the three of them could have been in conversation the night before. Shiv’s conversation with Tom earlier in the episode also seems to hint at Shiv wanting some kind of reconciliation in their relationship even though Tom seems to (rightfully) not want anything to do with her anymore. Tom asking Shiv to then take a car home with him after the deal goes through could just be a power move on his part, but it could also be further proof that they had some sort of talk about Shiv changing her vote in his favor.
Though if you truly want to give Shiv the benefit of the doubt, it could be argued that she didn’t fully change her mind until the sibling’s fight outside of the boardroom when Kendall yells “I’m the eldest boy” to try and convince Shiv that running Waystar is his birthright. If her mind wasn’t already made up by this point, seeing her older brother once again prove his immaturity could have easily changed her vote.
In an interview with Variety, finale director Mark Mylod seems to argue that there’s not necessarily one specific moment that causes Shiv’s vote to change, but rather a cascade of moments that push her to this choice. Mylod says:
“Once they get back to New York, and the closer it comes to that moment where she has to raise her hand, Sarah knows instinctively to feel those layers — shedding those defenses those delusions. When they’re in Logan’s office, she sees how Stewy and Kendall are a couple of cronies together, and she sees how sidelined she could be by that relationship. When Kendall puts his feet up on Logan’s desk, there’s that visceral appalled reaction we see from Sarah. As she walks down the hallway into the boardroom, in the moment we might interpret it as determination and focus. But on a second view, we might see it as doubt. As Kendall struts around the board meeting as if he owns the place, we cut to Shiv to see the erosion of that lie she is telling herself. By the time she actually has to raise her hand, it’s as impossible as it is for Roman in Season 1.”