While takeing off form PDX the other day (en route PDX-SFO) I noticed several UA aircraft parked a distance from the terminal along with the usual 6-7 aircraft at the gate. The aircraft appeared to be 737s, including one in the new colors, an A320, a few 757s. What are they all doing here? It this normal or are they undergoing some kind of MX? Any comments would be appreciated.