Hey there Eire Cailín,
Thanks for reaching out to Apple Support Communities. I understand Siri is turning on by itself. I’ll be happy to help with this.
Do you have any devices connected to your iPhone via Bluetooth? Go to Settings > Bluetooth and turn it off. Test your iPhone out and see if the issue returns. If it doesn’t, there may be a Bluetooth device connected to your iPhone that is related to this. If you have a case on your iPhone, remove it as well to make sure it’s not related to this.
If the issue continues, some other steps that may help would be to restart your iPhone:
Restart your iPhone
Make sure it’s up to date:
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Let me know if these steps help.