Okay so there are a lot of reasons that you might pass out donating plasma. You could have low blood pressure which they tend to check for the plasma donation center but some people lie on the test and say that they have perfectly fine blood pressure.
You could also have celiac disease or diabetes, which tend to cause low blood pressure at random times and make you pass out while donating plasma.
They also say that smoking causes low blood pressure. This is why they tell you not to donate plasma for one hour after smoking cigarettes. But I don't know if that's true or not, or just a lie from those PSA antismoking commercials.
Other Causes of Low Blood Pressure
As for other things that cause low blood pressure, there's anorexia, and not eating for a while. If you take low blood pressure medication with hypertension, this can cause low blood pressure, but they tend to check for that at the plasma center before you go.
Those are all the things I know about that cause low blood pressure. I guess there's also hard drugs, downers, barbiturates like morphine, heroin, and alcohol. So, you know, just don't do any hard drugs like a day before donating plasma, and you should be all right.
So yeah, basically, the reason for passing out during plasma donation is low blood pressure, and most of the causes of low blood pressure they screen for before you even go and donate.
So if you don't lie on the test, and you don't have any diseases that you don't know about, you should be fine donating plasma, and not pass out. If a keeps happening though, you should probably get to the doctor to test for some hidden underlying medical condition, because that's not really normal to pass out during plasma donation.