I know that this is by far the least of anyone's problems when going through the withdrawal process, but does anyone know why withdrawing from certain meds causes sneezing?

I am currently working on getting off tramadol (for the second time) and as you know it sucks. I understand why all the other crappy things are happening to me, but I know that within a day or 2 I will be sneezing like crazy, and making my coworkers wonder, and just wondered if anyone had any ideas.