I wonder if his wife/ex wife/girlfriend works in that building.
according to this source:
Joseph Andrew Stack has been identified as the pilot who crashed into the Austin Government building. Reports are surfacing that prior to taking off, he set his own house on fire, plus he had an ongoing feud with the IRS – the government agency located in the building in which the plan crashed.
http://www.mediaite.com/online/joseph-andrew-stack-identified-as-austin-crash-pilot/
Who knows? Could have been a random targeted act of intentional suicide/murder.