The car is a 2002 model. Last few months it began locking doors when I put the car in reverse to back out of drive. what causes this?
the car doesn't unlock when put in park. I have to manually unlock the car. can this setting be changed?

Oddly enough, Lexus has this 'personal settings' option that gets programmed exclusively by them. You'll need to bring the car to a Lexus dealership and let them know how you want the settings programmed...

Its done through LPS (Lexus Personalized Settings) I don't know why it just started doing this but LPS is programmed at the Dealership through the key. This setting can be changed.

Some of the sensors or other electronic things may be malfunctioning or wiring may be shorting some things. I recommend that you take it into the dealer and have it checked out. This may cost you a little but at least you have the sense of security that Lexus is fixing your car and that they will do things right.

most newer car have this feature. when you put the car in gear it locks the doors. if it wasn't b4 then somehow this setting was reset, ie power surge ect

NO! NO! NO! You can change the settings without taking the car to the dealer. If you have the manual that came with the car, it will tell you haow to change the settings. There are 3 settings you can set it to lock at a certin speed, unlock after you put the car in park within 10 seconds of turning the car off, and locking after you turn the car on. I set mine to unlock after 10 seconds after the car is set to park and the key is taken out the ignition. If you don't have the manual I will post the instructions