The 2 could be related for a number of reasons:
1. Higher wattage bulb puts out more heat. The insides of the casing get hotter than designed, warmer air gets thinner inside to maintain same interior air pressure (temp is related to pressure at a fixed volume ) I.e. most of it gets out. The casing then cools down, drawing in damp air to maintain atmospheric pressure which then condensates inside the housing.
2. Hotter operating tempuratures, casing slightly ruptures, letting in damp air more easily, again condensing when the casing cools down.
3. Constantly swapping out bulbs, a seal hasn't been replaced quite right and a higher amount of damp air than normal has gotten in.
Am I right in thinking you didn't have a misting issue before you started swapping out bulbs?