Niobium (V) Fluoride becoming sticky
From the properties block on the Wikipedia page for $\ce{NbF4}$:

Niobium (V) fluoride is hygroscopic - it absorbs water from the atmosphere. This hygroscopicity is your main problem. When the powder absorbs water, it probably is becoming sticky. 

The added water is probably also providing the extra mobility the ions need to crystallize from the powder. Niobium (V) fluoride forms monoclinic crystals, which on a very small scale could look like hair or needles.