M.M.
My 8 month old has done this several times too--it can be QUITE scary! I think two different things have been happening with him: the first is teething...and there isn't much I can do about that...sometimes we give him a little teething gel, other times we just comfort him back to sleep. A few times he has been HYSTERICAL and when I pick him up he is very stiff and his eyes are still closed...I think he is having nightmares, although at this age, I can't imagine about what! I read up on it and they said not to rush in a pick him up to try to wake him...which I was doing...now I will go in there, rub his head or back gently, and sometimes will EASE him into waking up...but they said it can be more startling to them if you rush in, pick them up, and try to get them to wake suddently...
Per the other comments: my son has an ear infection that would wake him up often enough as well.
Good luck!!!