My 13 month old baby wakes up once or twice every night...sits up or stands up in her crib and cries out. What could be causing this?
She can't be having nightmares every night. My wife has not weened her from night feedings, so some of the wakings could be due to this.
However, sometimes I can actually talk calmly to her and help her lay back down...and this will put her right back to sleep.
What causes these night wakings? Could she be sleepwalking?
You should edit your question to mention the age of your daughter. You mention nightly (breast?) feedings but also that she can stand, so I'm going to assume she's between one and two years old:
It doesn't have to be nightmares that wake her -- the dreams can also just be regular positive ones. You know that dreams can feel very "real" and even if they are positive that would still be a very confusing situation to a small child.
This would explain why you can talk to her, and this communication "resets" her perception of reality so that she can go back to sleep. This also explains why you can get her back to sleep without feeding -- the waking is not driven by hunger.
I'm guessing that nightwalking doesn't look like this, and/or that it doesn't occur at her age. I would not worry about this potential situation until she has a bed that she can get out of and move around your home.
Even the perception of what is a nightmare is different for children. Some infant I knew was waking up for some consecutive nights because "there was a turtle in her bed", and that turtle was "using some of her space, and turtles shouldn't be in my bed". And even being able to explain this after few nights, when she was dreaming and... bored about the turtle, she preferred to scream or cry. Specially because dreams (and nightmares) aren't always a complete package, many times you can only find them to be logical after trying to explain it to someone else.
Ok, so imagine that to a child. Reality and dreams are completely mixed, and he/she doesn't even know how to rationalize about it... I would cry, too :)
One thing that might help is investigating and perhaps changing something in his/her environment: does he sleeps in the crib, or in a different room, so that when he wakes up he's lost? Is the room completely dark or there is some light to help him seeing where he is? Is there some strange shadows, caused by the light? Has something changed recently, like a new toy, that is too exciting, that could always appear in his dreams ?
