When a gravity-affected block recieves a block update which comes from the sides or from the top, it will end up realizing it is in an invalid state and fall.

While this has been the case in-game for a very long time, it can be argued to be unintentional as of 1.13: several blocks were made to not update and break if receiving a block update unless that block update specifically happens on the face that block relies on for support. In the example shown, a suspended torch does not break when a block is placed directly next to it, but is when a block that cannot support it is placed directly below it. This is intentional as per MC-129127, so it not being the case for suspended blocks is inconsistent.

I can see this being made an exclusive special case for gravity-affected blocks however, due to the dangerous "cave-in" mechanic allowed by suspended gravel veins.