After a run of wins, risk appetite jumps. The gains have not yet been mentally absorbed into your net worth, so losing them feels cheaper than losing your original stake. Casinos rely on this; so do fast-moving markets after a strong week.
The practical outcome is that traders take their largest positions right after their best stretch, which is often when conditions are most extended. Give-back is the standard result, and it feeds profit-giveback and position-size-creep.
Sweeping profits into a separate account on a schedule, or simply recalculating risk from total equity each morning, removes the label that makes the money feel free.
Related: mental-accounting, hot-hand-fallacy