Apple, in a statement given to Rene Ritchie:

Updating some iOS devices (iPhone 5s and earlier and iPad Air and earlier) to iOS 9.3 can require entering the Apple ID and password used to set up the device in order to complete the software update,” an Apple spokesperson told iMore. “In some cases, if customers do not recall their password, their device will remain in an inactivated state until they can recover or reset their password. For these older devices, we have temporarily pulled back the update and will release an updated version of iOS 9.3 in the next few days that does not require this step.”

There’s also a support article.