It's a rather surprising move. Many believed that Karlsson would spend the year in the AHL improving his game, and keeping his pay off the Sens' books. I believe a combination of factors led the team to make this decision and start the clock ticking on the talented Swede's second contract.
Why now?
Karlsson has been doing well in Binhamton, I see from his stats. Eleven assists in 12 games puts him among the team leaders (though most of the Baby Sens have played 20+ games). It looks like his level of talent deserves to be in the big leagues. (Looking down the rest of the B-Sens roster, I see a lot of long-ago Ottawa draft picks... most will turn out to be AHL lifers. Ouch.)
Recent wins may have emboldened Sens management to 'go for it' this year and bring Karlsson along for the ride... and by 'it', I mean 'making the playoffs'. Playoff prospects are looking good at the moment... let the good times roll.

What about our former golden boy?
And Brian Lee? Well, he has fallen down the depth chart for sure, but he's still on a two-way contract and could still prove to be valuable for the Senators down the road. Don't count him out yet. Many have said that he's likely to be traded in coming weeks, but the Sens don't have a ton of cap room ($1.9-million according to nhlscap.com) to accept new contracts.
WTF Jersey
PS: Did anyone see the Baby Sens' Thanksgiving jerseys? I think we have a new winner for Worst. Jersey. Ever. The bad-ass Senator guy is wearing a pilgrim hat! WTF!