What causes a beach to slope downwards?

if there are waves the waves wash the dirt into the water. if not, the nearby water causes the ground surrounding it to be unstable and loose because the water gets drawn into the ground underneath. if you dig down near a beach you can find water. so if the ground is loose and unstable it easily gets drawn into the lake. another reason could be is water gathers in low areas because of gravity. which is why there's a lake there in the first place. so the ground could have just been that way in the first place.

Mainly the erosion and rise in water level.When the tide recedes back to normal it creates an abrupt slope which may regain old shape after some time.

A lake is a big hole in the ground that is filled with water. The beach is simply an extension of that hole that is not covered with water at the moment.

Gravity (water runs downhill eroding as it travels downhill) The moon (causes tidal action moving water to cause erosion) The Sun (which causes wind which creates waves that help to erode the sand )