Blood has hundreds of different proteins in it, each with its own function. The function of many blood proteins remains unknown.
A common blood test (called "serum protein electrophoresis") separates various blood proteins according to their electrical charge. The alpha-2 fraction of globulin is one of these groups of proteins. It tends to increase in any condition in which there is inflammation in the body.
There is no single explanation for why you may have had a high blood protein or alpha-2 globulin; there are hundreds of potential explanations. It could be something as minor as a cold or as major as pneumonia. Certain types of arthritis, blood diseases, and stress can also increase this group of blood proteins. It's even possible that despite extensive evaluation, no explanation is found.
It's not clear to me why you would have been deferred for plasma donation due to an elevated alpha-2 fraction of globulin. Perhaps there was concern that you had an infection that could be passed to the recipient of your donated plasma. It's also not clear to me why this test was performed in the first place. It is not routine to perform a detailed protein analysis prior to donating plasma. Contact the medical director of the facility to which you donate and ask why the test was done and what you should do about the results.
Then, talk to your doctor about any symptoms you have, have a physical examination and, perhaps, additional tests to assess your overall health. If you had a minor infection to account for the elevation in the blood protein or alpha-2 fraction of globulin, the test result may be normal when repeated.
In most cases, when someone has a serious or important cause of elevated alpha-2 fraction of globulin (including severe infections), they know that they are ill and the finding is not a surprise. So, if you are feeling well (which should be true if you are donating plasma!), it's quite possible that the test result is of no importance at all.
By the way, thank you for donating plasma. Donated blood products save lives and since anyone could require donated blood or plasma, everyone should be grateful for the generosity of blood and plasma donors.