It seems part of the current rumble among the leading U.S. carriers that began with the flurry of new unlimited plans launched. (techtimes.com)
(A January update found that the margins had shrunk, though Sprint and T - Mobile heavily promoted new unlimited plans in the time between.) (businessinsider.com)
During times of congestion, your data may be temporarily slower than other traffic (only after 22 GB / line / mo on New Unlimited Plan & Beyond Unlimited). (verizonwireless.com)