Unfortunately for Liljegren, Sandin has been ahead of him on the depth chart for some time now and given the uncertainty with Muzzin (he left Monday's game briefly after taking another hit), the Leafs couldn't afford to take a chance in sending Dermott down and losing him on waivers. The Leafs could have also sent Brooks down to the Marlies, but because the Leafs claimed him, they would have had to put him on waivers again tomorrow in order to send him down. Clearly this was something they did not want to risk again.
During his post practice media availability, Keefe hinted that the defensive pairings in practice could be different tomorrow and the move to send Liljegren down confirms Keefe's statement. Dermott was the 7th defenseman at practice today with Sandin and Liljegren playing as the third pair, but it appears that Dermott will draw into that third pairing along with Sandin now that Liljegren has been sent down to clear cap space for Adam Brooks. In addition to clearing cap space for Brooks, who is on IR currently, the Leafs will also accrue more cap space down the road because Brooks' cap hit (725K) is less than the league minimum. This would allow the Leafs to recall Liljegren by March 2, unless other moves (possible Ritchie/Dermott trade?) are made allowing them to do so earlier.