High humidity. If an air conditioner is leaking, it's drain line might be clogged.
[Sc] This may also be caused by a cold air return filter that is either clogged or is too restrictive. Filters that are able to trap very small particles have a lower flow rate and can restrict air flow. This slowing of the air flow across the evaporator means that moisture in the air is more likely to accumulate and, in some cases, freeze and cause damage.