The 6DE seats are blocked for handicapped accessible seating. You'll find that certain bulkhead seats in economy cabins on just about all UA aircraft as well as UAX aircraft are blocked off for this reason.

The other seats are not handicapped seating but I've found that UA does that on most of their aircraft as well. They hold a few seats towards the back until close in to departure.

All those seats including the handicapped seats (if they haven't already been assigned) get released at 24 hours prior to departure.