Moist air is condensing (barring any oughtright leaks) It is most likely due to air leakage from house to attic:
- Ceiling to attic is leaking moist air, condensing. Solution: air
sealing of all penetrations (light fixtures, ducting, electrical,
gaps in construction)
- Insufficient insulation, combined with air leakage allows
condensation to occur. Vapor barrier (VB) insulation (batt with
paper facing) should NOT be placed OVER any existing insulation,
creating a trap. If used, the VB is placed against the heated side
in winter. Do not use poly for a vapor barrier. The assembly must
be able to dry in either direction, should condensation occur.
Update RE can lights
Recessed cans are notorious for leakage. If they are IC rated (designed for direct insulation contact) they can be air sealed with a ASTME 136 rated firecaulk directly. If they are non-IC, a XPS box can be built that meets the clearance requirements of the fixture (insulation must be a minimum of 3" around a non-IC fixture (NEC 410.66)) Many commercial products exist for covering cans.
Video: Airsealing recessed lighting by Green Homes America
I've seen this condition develop when a bath vent fan was dumping air directly into the attic and it saturated the insulation and it grew mold down through the drywall.