Dry solder joints - what exactly happens?
During the transition from liquid to solid, the solder must briefly be in an intermediate state. Kind of a slushy state where it is not solid, but also does not flow by itself. If the pin moves while the solder is in that state, then the motion of the part will create a hole or tunnel in the solder larger than the pin.

Then the solder solidifies fully.

So now you have a pin inside of a solder hole, with clearance all around it. Depending on whether it moves or how much it moves, it will come in and out of contact with the walls of the hole. In other words, it will have intermittent contact.

Depending on current and voltage through the pin, intermittent contact can lead to heating (due to high resistance) and electrical arcing (due to breaking circuit while current is flowing) which can lead to additional failures.

For a low-power, low-voltage logic connection, it can lead to intermittent erroneous behavior because the signal may not be in the logic state it is supposed to be in.