Mr_Mike_O was on the right track when he said the 49ers' receivers are getting tackled quickly. By then, Doug Clawson of ESPN Stats & Information had already sent an email showing the 49ers' yards-per-catch numbers over the past two seasons.
49ers Yards After Catch (per reception)
Across-the-board comparisons are problematic because the 49ers are running a different offense. They also used multiple quarterbacks last season, with Troy Smith making six starts. One of their receivers, Braylon Edwards, spent last season with the New York Jets.
But this is one aspect of the 49ers' offense to monitor as the season continues. The chart shows yards-after-catch figures for Josh Morgan (injured reserve), Frank Gore, Vernon Davis, Michael Crabtree, Delanie Walker, Edwards and Ted Ginn Jr. All the figures are down from last season.
Overall, the 49ers averaged a league-leading 6.1 yards after the catch last season. That figure is down to 4.9 this season (25th).
It's unlikely all these players suddenly lost breakaway ability at the same time.
Efforts to get rid of the ball quickly could play a role. Also, running backs tend to rank high in yards after the catch. Gore's receptions are down from 46 in 11 games last season to 14 in nine games this season. That could be one factor.
I don't have a definitive answer, but if you've got theories based upon how the 49ers are playing offense, please share. I'll ask around and pay closer attention to this aspect of the game.