By default, Ubuntu has been set up to respond to two finger taps as a secondary / right click. Ah there's nothing like trying to alienate Windows users eh?
To fix this behaviour, you need to install the Gnome Tweaks tool.
You can do this in the terminal with:
$ sudo apt install gnome-tweak-tool
Or just hit the Windows key and type 'tweak' and select Gnome Tweak
Once installed, run the app, go to Keyboard and Mouse and chnage the Mouse Click Options to Area.
Boom! Right click is behaving like it should :)