The mechanical consequence of many traders placing stops at the same obvious levels, producing a wave of simultaneous aggression when price reaches them.
Full Explanation
Prior swing lows, prior swing highs, round numbers, and session boundaries all attract not just your stop but a large number of other traders' stops. When price reaches one of these levels, all of those stops trigger simultaneously — flooding the market with a wave of aggressive orders at the same moment. If opposing liquidity at that moment is thin, the wave pushes price significantly beyond the level before it is absorbed. This produces slippage that is not a broker error — it is the mechanical consequence of stop orders sharing the same address. Awareness of clustering informs where to place stops relative to obvious levels.