It's partially related to the speed of a mo-cap tackle animation playing out versus the speed of the running animation. It's also related to the magnetic force-field effect which results in players locking onto another before actual contact occurs. If a defender breaks his block and goes for the tackle, then your ball carrier has been "contacted" when that player is within 1-2 feet and will magically slow down to take contact.
These are the kinds of issues that gamers complain about the loudest when asking for some form of "real-time physics".