Why does my Kenmore model 721.63352300 turn on when door is opened ?

This is normal. Kenmore microwaves will stay running with fan and light on for a short time after food is cooked. The reason is to air out the microwave after cooking. Things like pop corn, Bacon etc. have strong smells after cooking. The ventilation helps to remove odors.

The door interlock is defective

Possible that the door lock is jammed, it can also be due to jamming of the door switch or the plastic lock being stuck on.
You need to remove the front panel , and reset the door assembly.

It is one of two things, a defective door switch, or the keypad control board has failed.

Check those three microswitches in the latch assembly.

This is usually caused by either a bad door switch, a loose door switch mount, or a problem on the control panel.

Even though a door switch clicks, it may still be bad inside. Its continuity must be checked.

Switch or mount trouble can be caused by slamming the door or by opening the door while it's cooking without hitting the Stop pad first.

If you someone you know decide to look into it, we have critical information on safety, disassembly and door switches at our site, and our link is at our listing here on FixYa: http://tinyurl.com/yzjozk

You can probably find an exploded view by entering your FULL model number here: http://www.searspartsdirect.com

A full Sears/Kenmore model number usually has 3 digits, a dot, then 8 more digits, such as ###.########

If you think the problem is on the control panel, which we repair these nationwide for $39.95 postpaid.

We're happy to help and we appreciate your thoughtful rating of our answer.

Your model number prefix of 721 means it's made by LG / Goldstar.

F9 code on an LG means either a bad door switch or a loose door switch mount.

Even though a door switch clicks, it may still be bad inside.

This can be caused by slamming the door or by habitually opening the door while it's cooking without hitting the Stop pad first.

If you or a friend decide to look into it, we have critical information on safety, disassembly and door switches at our site, and our link is at our listing here on FixYa: http://tinyurl.com/yzjozk

You can find helpful exploded view diagrams and order parts by entering your full model number here: http://www3.sears.com (the 3 is correct)

We're happy to help and we appreciate your thoughtful rating of our answer.