Volatility clustering is the well-documented tendency for large price moves to be followed by more large moves, and quiet periods by more quiet — volatility is "sticky," not evenly spread. It's one of the most robust statistical features of markets (volatility regimes).

Why calm persists

In quiet regimes, low volatility encourages more leverage and risk-taking (it feels safe), which further dampens moves — until it doesn't (low VIX ≠ low risk). Calm feeds calm, building fragility underneath a placid surface.

Why chaos persists

When a shock hits, several forces amplify and extend it: fear spreads and feeds on itself (the fear and greed cycle), deleveraging forces more selling that begets more selling, and dealer hedging in negative-gamma regimes mechanically amplifies the moves (how dealer hedging moves price). One big day genuinely raises the odds of the next.

"It's been calm for weeks" is a description, not a safety guarantee. Volatility doesn't average out day to day — it bunches, and the bunching is where risk hides.

What it means for you

Clustering is why you size down and stay alert after a volatility spike (more likely coming), and why a long calm stretch warrants respect, not complacency (expansion and contraction). It's also why realized vol is somewhat forecastable in the short run — a fact the whole options market prices around (the volatility risk premium).