This is mostly hearsay (supported by things the WGA said, so I trust it), but not all members of the AMPTP were happy with the strike going this long and wanted to negotiate a lot sooner.
Earlier this month there was talk of some members leaving so they could negotiate new contracts independently. This might have lit a fire under the ass of the holdouts, since the AMPTP losing someone big like Disney would significantly weaken their own negotiating power.