She could have figured that at Momās house. Or anytime before they get to the board room. For her to come to that conclusion only at that moment is a Roman move. Itās nearly exactly what Roman did a couple of seasons ago when he went into the board room intending to vote against Logan and then flaked out at the last minute because he couldnāt bring himself to do it.
Itās not wrong for Shiv to have a bit of Roman in her. Itās just weird, because she has always been portrayed as being nearly the opposite of that. Itās not so much that she went against Kendall as it was the last minute, flaky nature of her decision.