Comment 40 for bug 1264795

I found a solution! At least for my case.

I have a 2015 MacBook Pro with Ubuntu 16.04 LTS.

According to these:
https://askubuntu.com/questions/456299/state-of-multitouch-gestures-in-14-04-unity
https://wiki.ubuntu.com/Multitouch

This happens, because sometimes the touchpad's 3 finger tap followed by 3 fingers hold is triggered randomly. You can disable this, by overwriting the TapButton3 option in synclient. Just like this doc says:
https://wiki.ubuntu.com/Multitouch/TouchpadSupport

I set it to behave like a middle mouse click (which is more handy because if you select/copy some text then 3 finger tap, it behaves like a paste in input fields).

Here is an Ask Ubuntu post that explains the process:
https://askubuntu.com/a/156545/323990

In short:
echo synclient TapButton3=2 > ~/.config/touchpad_settings
chmod +x ~/.config/touchpad_settings
Then run this script at startup. Searching for "Startup Applications" in the dash then add a new startup program with the command: "/home/your-user-name-here/.config/touchpad_settings". Don't forget to replace the "your-user-name-here" part in the path with your username.