UA839 This Morning.
 

I was out spotting today, and as UA839 taxied down to the holding point of 34L, I was just leaving and heard they we're having an issue. They ended up taxiing on the runway and vacating onto Lima then back onto Alpha, and then took off, but as I was driving away from the airport, I couldn't hear the finer details.

Does anyone know what the actual problem was?