Can wearing guard for bruxism cause a sudden onslaught of multiple cavities?

Not likely.: Bruxism, or continual or repeated grinding of teeth, is a serious problem that can lead to dental problems, jaw problem, muscle disorders involving mastication (chewing, and using jaw), TMJ syndrome. Wearing a guard at night to avoid clamping teeth down during sleep, is one approach to this significant problem. Muscle relaxation, and any approach leading to relax these muscles is important.

Bruxism guard/cavity: Wearing a bruxism mouthguard will not cause cavities. There is no such thing as a "sudden onslaught of multiple cavities." decay develops over time. The exceptions to this are people undergoing head/neck radiation, meth addicts, and people with sjogren's syndrome (chronic dry mouth).