Hi Layla it's either a faulty home button or faulty dock..
If you release the screen from the frame so the home button is not pressing against the docks gold prongs see does it still activate Siri. If not it's most likly the home button that's faulty..
If it activates Siri when the screen is lifted up of the frame then it's most likely the dock that's faulty..
You can unplug the dock if this is the case to rule out hardware damage