It could have been any number of things: including medical stuffs that were not time critical (organs and such), hazmat, live animals, or any number of other things.

As to why: it's entirely possible that the crew did not know about the specific cargo until contacted by UA dispatch, and the thermal printer in the cockpit might have been inop.

The problem here is that the number of possible things is so large, you may never know for sure unless someone with access to UA's internal systems can dig it up for you.

But the speculation is always fun .