In the winter humidity levels are lower in general simply because cold air holds less moisture in it than warmer air. Buildings that use a forced air heating system make the air humidity even lower by burning out any water vapor that did remain in the cold air. When the humidity is lower it makes it easier for these hazardous dusts to disperse in the air, and can lower the temperature needed for ignition.
When the heating system is running during the winter the occupants of the building will be sure to keep doors and windows closed to conserve this heat. In warmer weather these bay doors or building windows might be opened to let in fresh air, or there will be fans running to maintain airflow inside the building. This extra airflow can keep dust levels from becoming too high to become hazardous, but the lack of extra ventilation in the winter can let dust levels increase.
Many common housekeeping practices in manufacturing facilities rely on water, but when temperatures drop too low cleaning with water can’t be used. This causes either the dust to accumulate until warmer weather when water-based cleaning can be used again, or other cleaning techniques that can be dangerous for combustible dust are used. Often, people will try to use compressed air to clean combustible dust in an area that previously used a water-based cleaning. However, this just results in a dust cloud, creating a dangerous situation for everyone involved.