In the last chapter sasuke is shown to have only one sharingan activated, this could have something to do with itachi's blood dripping out of sasuke's left eye a few chapter ago. Lets go back to the chunnin exams in the forest of death, orochimaru gave sasuke a curse seal that enabled sasuke to forcibly draw out more chakra, this later came in very handy for sasuke and its the reason he was so overly powered. Now in the last chapter we can assume that after the fight with itachi sasuke has little or no chakra left, he used his most powerful techniques and he was reduced to using regualar weapons as his last defense. So my theory, sasuke is shocked when tobi removes his mask, but to activate his sharingan in the midst of a surprise and having little chakra seems improbable and he only activates one eye, could this be some kind of power given to sasuke by itachi in their final battle.
in the naruto world people usually pull trump cards out of nowhere during moments of shock or deep desperation for example, yahiko unleashed his rinnegan unwillingly, sasuke activated his sharingan for the first time in a desperate moment (same with obito), naruto succumbs to his kyubi chakra during intense emotional destress. So this could possibly mean that sasuke is not trying to activate his sharingan but it activated a power a new power (their could be more to sasuke's one sharingan) of its own accord from the result of sasuke's shock.