It started with DC's Dejan Jacovic wrestling down forward Kenny Cooper in the penalty box. Cooper did a little stutter step and then hit the kick, which United keeper Bill Hamid saved. However, the linesman (bravely but rightly) said raised his flag because Hamid had come off his line early. Cooper was awarded a retake, did the stutter step again, Hamid made the save again, and the linesman (bravely but rightly) raised his flag again because Hamid came off his line again.
Before the third try, Portland coach John Spencer waved Cooper off, and Jack Jewsbury knocked the PK home. Here's an MLS report on the game, including video of the odd sequence.
With the result, Portland remains three points behind Seattle in the MLS West standings, although with three games in hand.