Plexiglas is statically charged through friction (peeling off the protective foil / cleansing). This attracts other materials such as dusts, loose pigments from a painting or even thin works of paper.
Static charging cannot be averted, but it can be removed or avoided.
1. Peel off both protective foils
2. Touch glass with one hand and a heating pipe or other conductive material with the other for around 10 seconds.
3. If static charging persists, clean one side of the glass with an anti-static cleaner. Wipe the surface completely (e. g. first vertically, then horizontally, not circular!) to stop the charge. If you clean the glass anyway discharging via the heater is unnecessary.