They're pursuing Sky now against Fox which may complicate the Disney deal since Disney may want Fox to obtain the rest of Sky (Fox owns 39%) as part of their deal and won't want to be a minority partner with Comcast.
The entire Comcast bid may have just been a plot to force Disney to spend so much extra that they have to sell something off to appease shareholders, and selling their Sky shares to Comcast seems like one way to ease the pain of their massive purchase.