Those buses are chartered by Disney Sports, aka ESPN Wide World of Sports. They routinely charter buses for events, which are run completely separately from the Disney Transport buses. In fact, most of the time Disney bus drivers aren't even told when and to which resorts the event buses will be running.
Other buses on property that are contracted to outside companies include the shuttles for College & International Program cm's, buses to Hotel Plaza at DTD, and Shades of Green buses.
During peak periods, including now, Disney Transport also charters buses to help out on resort service, because Disney simply does not have enough buses to get the job done. They come from Mears, American Coach, and lots of small companies.