This is a new problem introduced by iOS 18. Siri apparently turns itself on unexpectedly. This is when you double tap on the bottom.
To turn this aberrant behaviour off, just turn off Type to Siri.
This is indeed an intended new feature of Apple Intelligence - introduced with release of iPadOS 18.2.
A double-tap of the Home Bar, seen at bottom centre of the iPad screen, will trigger an Ask Siri query using keyboard input. This behaviour is documented within the iPad User Guide:
Use Siri on iPad - Apple Support
Answer for “Siri keeps randomly turning on by itself”
