Siri reacts when "Hey Siri" is heard. I can also activate my wife's iPhone, although her voice is significantly brighter than mine. If you find that others activate Siri when you have your phone in your pocket, bag or with the screen facing down, you can disable Always Listen for "Hey Siri".
The function is disabled by default, but if you have enabled it at some point, you can disable it here:
Settings → Accessibility → Siri → disable Always Listen for “Hey Siri”
You can also disable Siri when your iPhone is locked. Go to:
Settings → Siri & Search → disable Allow Siri When Locked