LH managed to operate the 07:20 and every other flight to Munich that day, but cancelled the 09:20. Call me cynical, but I think they may be using the weather as an excuse to cancel flights which are not heavily booked.
The reason LH managed to operate the 07:20 departure (and the 06:40 one prior to that) is that both of those are nightstopping aircraft.
The first LH due in on Tuesday morning (08:30) was one of that day's 36 cancelled arrivals, so there wasn't an aircraft to operate your 09:20 departure.