Concise answer
Washing can release hairs that had already completed their growth cycle, especially when several days have passed between washes. The visible amount does not by itself identify the cause of shedding.
Loose resting hairs collect between washes and may come away together during shampooing or detangling, making the loss look sudden.
The pattern over several weeks, changes in density and recent illness, stress, nutrition or medicine history are more useful than one wash-day count.
When evaluation may help
Arrange assessment if increased shedding continues, the scalp becomes more visible, bald patches appear or there is itch, pain, scale or redness.