The defense was mainly to blame for the Lions' collapse
The Lions played dominant football on defense for nearly three quarters on Sunday. Then it all went downhill after the Lions took a 14-0 lead. Suddenly Nick Foles was able to complete passes to open receivers, and suddenly the run defense, which had been dominant for most of the season, completely fell apart. Foles finished with 179 yards through the air, along with 1 touchdown and 1 interception. LeSean McCoy, meanwhile, rushed for 217 yards and 2 touchdowns. As a team, the Eagles had 299 yards on 46 carries (6.5 yards per carry) and 4 touchdowns.
With the offense playing so conservative because of the snow, the Lions relied on their defense to protect a 14-0 lead, and the defense responded by completely folding.
The offense deserves some blame as well
I don't fault the Lions for being so conservative considering how well the defense was playing. However, the offense doesn't escape blame in this debacle. Their general inability to move the ball outside of a few drives, especially in the second half, helped lead to the defense being so gassed by the end of the game, and their inability to protect the ball prevented the Lions from pulling away early in the game. Yes, the weather stunk, but the Lions lost 3 fumbles and had 7 fumbles in total. The Eagles' only turnover was an interception, and they only had a single fumble, which Foles recovered. Bad weather or not, the offense's issues with protecting the ball continued on Sunday.
The Lions have found their returner
On a positive note, Jeremy Ross was sensational as the Lions' returner on Sunday. He had 4 kick returns for 172 yards, including one that went for 98 yards and a touchdown. He also had 2 punt returns for 71 yards, including one that went for 58 yards and a touchdown. He combined for 243 yards and 2 touchdowns on returns, outgaining the Lions offense, which had only 228 total yards and 1 touchdown, in the process. I think it's safe to say Ross is here to stay as the Lions' returner.