The Ancient One...
*** SPOILER ***
We, as the audience, were privy to the fact that Dr. Strange viewed over 14 million possible futures (how could he even keep track of that....yikes) and that they only won ONE. Therefore when Strange turned over the stone willingly, we were to understand this is the way it had to unfold, the only future in which they were victorious is one where he gives the stone to Thanos. (I'm surprised Stark didn't realize that immediately).
The Ancient One wasn't privy to this but the instant she was told by Banner that Strange willingly gave the stone up, she changed her mind and gave it to him.
Are we to believe that she also instantly knew that Strange must have viewed futures and would have only given the stone if it were imperative that this occur?
Does anyone else have any other theories as to why she so quickly had an epiphany when told that Strange (who she wouldn't meet for 5 years) gave the stone to Thanos?