Most washing machines have a timer that prevents you from opening the hatch just after the washing cycle ends. Instad you must wait for the timer to go off, usually a minute or two, before you can open the hatch.
Why? Would letting the user open the hatch immediately after washing ends pose any safety or other issues?
Many machines use a bimetal to lock the door, a stupidly simple design. Metal heats up, bends and triggers the lock. It takes a while to cool down which is why it'll take a while to unlock even if you unplug the machine.
In case you'd like to see bigclive explain it in detail: https://www.youtube.com/watch?v=PIm7q_U3UEM
It's so that the machine elves have some time to hide!
On a serious note, I found this explanation here:
Washing machines must have some way in which you can lock the door closing mechanism when the machine is started up and then unlock them with a certain delay (normally two minutes) after the current has switched off via the program or on/off switch, in order to ensure that the door cannot be opened while some of the components are still rotating initially (in particular the motor and the drum of the spin-dryer).
Washing machines have a bi-metal strip inside the door lock which is heated by PTC Heater (resister) when live and neutral are activated on to the pcts it heats up and bends the bi-metal strip which then moves the arm to activate the common terminal and push a pin into the closed door to lock it in place. Once this has happened (usually a second or so see video) the power then can flow through to the common wire, and therefore on to the rest of the machine allowing it to start.
