Why my car alarm is generally activated by itself?

I f your meaning is, Why does it arm if you don't use either the door lock or the remote it's because of a bad microswitch in the drivers door lock. The car thinks the door is shut and locked if it isn't. you'll have to replace the drivers door lock assy to make it work properly.

25 yrs experience working on VW's

probably hooked up wrong. or maybe a short in it.

It may be set where it is too sensitive and people to get near the car are setting it off.