Why is my AC blowing from this pipe and freezing up periodically?
Sounds like the condensate drain. The process of cooling air causes condensation to form inside of the AC and it's drained out through these lines. There's usually a small P-trap off the side of the AC unit (at least on HVACs for homes) that collects enough moisture to seal airflow (just as a trap under your sink keeps sewer gases from coming into your home). You have to periodically put something in this trap to keep it clean from mold (I use a cap-full of bleach, others suggest something like vinegar to be better for the environment).

As for why your AC unit is freezing up, I'd check your filters. If they are clogged, the AC may be blowing out the condensate trap and freezing over due to insufficient airflow.