Westerners are spiritual 2 year olds. We're walking around, putting stuff in our mouths, saying the dumbest things, breaking stuff and generally exploring the world around us. The Universe is a very complex place and despite our social delusion that magick and spirits are made up fantasy, they are very real and most of us are ignorant about how they work.
Ouija is a really nifty ritual. It is like a psychic go-cart (it helps us go places we'd have to spend a lot longer without it). All magick rituals amplify the results of our efforts, but (like most good magicks) Ouija is both powerful and simple.
Most of us don't know how to use our telekinetic powers. Ouija boards are both good techniques which help us experience Telekinesis by empowering forces outside ourself. The easy ways for a spirit to move stuff are to use your body or your energy to do it. Explaining the “your body” here will probably just terrify you, but the “your energy” is pretty straight forward. Our ability to move things with our mind is mostly untapped and because of that we can easily be tricked into doing it by another spirit.
Notice that Ouija commonly needs another person? That's because when we have more than one person doing something we form a “group consciousness.” Group consciousnesses are the source of pack mentality, shared illusions and a few other nifty spiritual techniques. In the Ouija ritual all parties give access to their telekinesis to the group consciousness which then uses it for cool things.
Scary seeming stuff can happen when one of the members of the group consciousness is afraid that scary stuff will happen. Their fears can infect the group consciousness which then starts doing scary stuff. That scary stuff scares other group members, and the whole thing can spiral downhill fast.
Unfortunately that was the hearts and rainbows version of scary stuff. The other version is when an actual unfriendly spirit gets involved. Most people doing Ouija don't know how to create safe sacred space and leave the door open for any spirit to join the group consciousness if it chooses. Depending on your luck (and a LOT of other factors) your summoning may call one or more spirits who want to help, mess with you, teach, hurt you... or all of the above. When a spirit who knows the rules joins the group mind, they can do things like take over the group mind, use the group mind to manipulate its members and form a link with group members.
Most of us won't get these worst case scenarios. We all have guardian spirits who make sure that other spirits don't hurt us too much and that we don't really screw up too badly. Sometimes our own guardian spirits may even do or say some scary stuff just to get us to back off and stop playing with stuff we aren't ready for.
If you do actually have something bad-ass (aka terribly fucked up) happen congratulations. That means you are probably one of the small fraction of Americans who have been called to study the spirit world and the rules of magick. This experience was your wake up call. You should probably find a teacher. Stumbling your way through this is great drama, but not a great study technique. You can find my contact information on the contact-information page.