On Day 1 why do some player's Current ratings drop from what they were at the end of last season?

After the SimBowl we age the players by one year and recalculate the Current Ratings. If they don't have enough Experience to overcome being a year older or they have enough injuries in their careers then their rating will drop.

This point drop is easier to gain back than the original development was. (IE they will regain these lost points quickly).