Alarm in the car reacts on voltage drop. That voltage drop is provided by putting wire onto ground/earth voltage. If wire is put onto ground - earthed, alarm will go off.
There are two switches in the boot. Boot lid switch and boot lock switch. If alarm is armed and you put your key in the key barrel and unlock your boot by key, boot lock switch will provide information to alarm that boot is opened with the key, assuming that is legitimate process and by that alarm will suspend its behaviour when boot lid switch voltage is dropped to ground by putting wire onto earth connection and will not triggere the allarm off when boot lid is opened.
However, if you open boot lid without putting key into boot lock then alarm assumes boot lid is opened by force and alarm will go off.
This means that if boot lid switch wire is earthed by coincidence or by error or by unwanted state, when alarm is armed alarm will go off.
