Not a doctor here - but realistically something in the salts reacted with the water - possibly even the temperature of the water breaking the salts down - and your brain made you 'smell' the closest thing that came close and it happened to be garlic.
Looking online it says that the smell/odor of garlic is mostly sulfur, and wisegeek.com lists this under powdered sulfur:
Cosmetic and Topical Uses
The powder can be purchased in its pure form and mixed with cream or bought as a ready-to-use ointment to put directly on the skin, usually for the treatment of acne, dry skin, or eczema. Traditional Chinese medicine has incorporated the powder into a number of topical skin treatments for centuries, including use as a mask believed to pull impurities out of the skin on the face, particularly the sensitive area around the eyes. The powder is also an ingredient in many over-the-counter acne solutions and facial scrubs.