Very common, and the front will probably start squeaking too, soon.
Be careful not to get grease/oil on the disks and don't touch them with you bare hands either.
Try degreasing the disks with a paper towel liberally soaked with isopropyl alcohol, then let them dry thououghly and go over them with a cheap microfiber cleaning cloth to remove any remaining film.
After that, just going over them with a cheap microfiber cloth (reserved JUST for disk cleaning) every few rides should help keep them about a squeak free as possible.