Large currency options expire at a daily cut, most commonly the New York 10:00 cut, and the dealers who wrote them hedge continuously. Where a big strike sits just above or below spot, that hedging tends to pin the rate to the strike as expiry approaches, because delta hedging sells strength and buys weakness.
Barrier options behave in the opposite way. Approaching a knock-out level, hedging can accelerate the move rather than damp it, and once the barrier trades the hedge is abandoned and the rate often snaps back.
Banks circulate daily lists of notable expiries, and these cluster on the round numbers that also attract stops, which is part of why the areas around big-figure levels are so often congested.
Example: $2bn of EUR/USD strikes expire at 1.0800 at the New York cut. Spot spends the morning within ten pips of that level, then moves 35 pips within minutes of the cut passing.
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