This is probably why.
http://www.mercurynews.com/49ers/ci_26221852/49ers-no-2-running-back-kendall-hunter-tears
49ers' No. 2 running back Kendall Hunter tears ACL
By Cam Inman
cinman@bayareanewsgroup.com
POSTED: 07/26/2014 10:00:26 AM PDT
0 COMMENTS| UPDATED: 9 DAYS AGO
SANTA CLARA -- Kendall Hunter, the 49ers' No. 2 running back since 2011, tore the anterior cruciate ligament in his right knee Friday at practice, the team confirmed Saturday morning.
Candidates to become Frank Gore's top understudy are LaMichael James, Carlos Hyde, Jewel Hampton and Marcus Lattimore, who's on the non-football-injury list but has been doing conditioning drills on a side field in training camp.
Hunter was entering the final season of his rookie contract. He was hurt catching a swing pass from Blaine Gabbert in 7-on-7, noncontact drills. Hunter's knee buckled as cornerback Perrish Cox closed in on the play.
San Francisco 49ers' Kendall Hunter (32) runs for 45 yards against the Atlanta Falcons in the fourth quarter at Candlestick Park in San Francisco, Calif. on Monday, Dec. 23, 2013. (Nhat V. Meyer/Bay Area News Group) ( Nhat V. Meyer )
Hunter ran for a career-low 358 yards last season. He had only 78 carries, just six more than his his injury-shortened 2012 season. He had 112 carries for a career-high 473 yards as a rookie, then followed with 371 yards in 2012 before his injury.
He joins several teammates recovering from ACL tears: Lattimore (non-football-injury list), linebacker NaVorro Bowman (physically-unable-to-perform list), guard Brandon Thomas (NFI), cornerback Keith Reaser (NFI) and fullback Trey Millard (NFI).
Thomas, Reaser and Millard all were drafted in May after their ACL surgeries. Last year, the 49ers drafted defensive lineman Tank Carradine in the second round despite his having undergone season-ending ACL surgery. Carradine now is making his training camp debut.
The 49ers haven't had much recent success returning players from ACL tears. Linebacker Darius Fleming retore his ACL a year ago in camp and was subsequently released. Kyle Williams returned last season from his 2012 ACL tear, but he was released at midseason.
Mario Manningham had a short-lived comeback last season from his December 2012 ACL tear. He debuted Nov. 10 in a loss to Carolina, and his comeback was cut short after six games (nine catches, 85 yards) when his knee became aggravated. He went on injured reserve Dec. 27 and missed his second straight postseason.
The 49ers also lost wide receiver Brandon Carswell to an ACL tear on his first day of practice upon being signed in June 2013.