The head of the U.S. communications regulator on Friday said he is asking all large U.S. wireless carriers to explain how they decide when to slow down download speeds for some customers, after questioning Verizon Wireless about such a plan. Verizon, the No. 1 wireless carrier, in July said the top 5 percent of high-speed data users on its older, unlimited data plans might experience slower speeds starting Oct. 1.