Why is "OK Google" using Skype to make calls?

[AC Question]

Suddenly, when I say "OK Google, call home" I noticed that it was using Skype to make the call. It has previously always gone to the regular phone app. I couldn't find a way to change this behaviour. Even if I removed all the defaults, it would just give me a choice of phone or Skype, but my choice wouldn't stick.
Finally I uninstalled and reinstalled Skype, and this seems to have fixed the problem for now, but I'm wondering if it might happen again, and if so how do you control which app "OK Google" will use?

Nexus 5 Lollipop

 

[B. Diddy]

Welcome to Android Central! Is this when you say OK Google directly to the phone, or when using Google Now over a Bluetooth headset?

 

[Avenger]

Same issue here when using my watch. How can I make it use the normal dialer?