It's definitely an interesting move. My guess is Adam has played out what will happen later in the show. He has probably figured that Ryan and Natalie are tighter and thus he would be next to go after Sheila!
I think he thinks that he couldn't win. If he put James up and James won the veto, he would be after Adam, whereas, if he saves james this week, if James wins HOH he will be on eof the last standing from his group.
I don't seeRyan or Natalie would use the power of veto, because they will naturally assume that Adam is now in an alliance with James, if they used the veto to save Sheila, one of them would go up against Sharon, and if either of them saved Sharon then one of them would be up, and deffo go, because it woud be Sharon and Jame's vote against the other one vote!
I think to be honest what Adam did made sense really. He obviously wasn't 100% sure that Ryan would take him through, and in fairness his bond was with Matty not Nat or Ryan, so he obviously feels he can make a better alliance with the guy who has no one else, lets say Sheila goes this week, Adam probably figures that he would be the guy in the middle ground and Nat and Ryan would want to take out James and Sharon and vice versa!