San Jose Earthquakes dropped the first leg of their semi-final to New York Red Bulls at Buck Shaw Stadium before downing New York at Red Bull Arena to qualify for the Eastern Conference final whilst Colorado Rapids edged Columbus Crew on penalties in the second leg at Columbus Crew Stadium.
In the west, FC Dallas held reigning champions Real Salt Lake at Rio Tinto Stadium after winning at home to go through as his Galaxy won away at Qwest Field before eliminating Seattle Sounders at home.
With most home sides not enjoying favourable results, Arena was lost as to why this was the case.
“Who knows? This is the craziest thing I’ve ever seen,” Arena said.
“You always think if it’s an advantage to play the first game, second game – who knows? All I know is that Dallas had a terrific performance against Salt Lake, San Jose against New York, and Colorado had a great effort to come back and get that goal against Columbus.
“Those teams did play well. Why the home field didn’t hold as well as people would have thought, who knows? I can never figure out any of this stuff.”