After a Pat Noonan goal put Seattle up one just before the first-half break, the Union exploded with goals by Sebastien Le Toux, Fred and Danny Mwanga in the second to earn their third win of the season.
The game started with very little action by either side, with lot of possession in the midfield mostly controlled by Philadelphia.
But Seattle (4-7-3) was the first to score against the run of play when winger Steve Zakuani played a ball into the Union penalty area to Fredy Montero. An errant touch by Montero went right to Noonan, who slotted the shot past goalkeeper Chris Seitz for his first of the season.
Seattle defender Tyrone Marshall took down Alejandro Moreno in the penalty area nine minutes after the break to give the home team a penalty kick, which was converted by Le Toux to even the score at one.
Seattle earned its own PK just before the hour mark when Montero was taken down by Cristian Arrieta, but Noonan blasted the chance right at Seitz and the score remained tied at one.
Seattle goalkeeper Kasey Keller made a terrific save off an Arrieta header following a Le Toux corner in the 78th minute, but Philly was able to get the go-ahead goal less than a minute later.
Le Toux played a low, hard ball into the Seattle penalty area that went through the legs of a number of defenders before finding Fred at the back post for an easy tap-in. It was the former D.C. United winger’s first goal of the season.
Union rookie Danny Mwanga added the third goal six minutes from time off another set-up from Le Toux. The Frenchman ran into the Seattle area and onto a ball played from the back before playing square to Mwanga. The rookie’s initial shot from point blank was saved by Keller, but Mwanga knocked in the rebound for his fourth goal of the season.
The Union (3-7-1) will aim to build on the momentum at Chivas USA on Saturday, while Seattle hopes to rebound at the league-leading Los Angeles Galaxy a day later in its next MLS fixture.