Welcome to lousy goalie night in the NHL!
Games between the bitter rival Pittsburgh Penguins and Philadelphia Flyers tend to be high-scoring affairs. That's in part because both teams take a lot of penalties, leading to a lot of powerplays, but it's also due to the questionable goalies both teams employ.
A combination of undisciplined play, bad defense, and shoddy netminding for the Penguins allowed the Flyers to open up a 4-1 lead at the end of the first period. The Flyers dominated in shots, leading 18-4 through the first 20 minutes, but even that doesn't fully excuse some of the soft goals given up by Marc-Andre Fleury.
As the period ended, Fleury was seen slamming his stick in frustration as he headed to the dressing room. There's an excellent chance he won't be in net when the second period begins.