Any idea who is behind all the free royalty free websites such as all-free-downloads, pixabay, pexels etc? It seems that Shutterstock is the main advertiser there but are they also the owners?

And also , where are all these free photos coming from?

Could SS's ads just be Google/whatever ads, triggered by keywords?

Shutterstock owns none of these. The sources are pretty easy to find - there are lot of photographers releasing a lot of decent photos into the public domain, and those sites aggregate the images and post them (since that allowed, with CC0 public domain license). All these site drive traffic to shutterstock for an affiliate kickback.

Simon Steinberger and Hans Braxmeier are the founders (and I guess still owners) of Pixabay.
I guess their plan is to keep growing until they get a multimillion dollar offer. 

I was a bit surprised on looking through Pixabay that there were images of trademarks, brands, identifiable models and buildings for which most stock agencies would, rightly or wrongly, demand a property release. How can they then say images are �free for commercial use�?

Well, they can�t and on following through to their FAQ https://pixabay.com/en/service/terms/#usage they �cannot guarantee� you can use images for any purpose as Pixabay does not require a model release, If there is one they can�t guarantee it�s legal.

For other trademark or third party rights no warranties given. �Use at your own risk�.

An interesting business model. Use whatever you like but if you get sued it�s your problem.

Any idea who is behind all the free royalty free websites such as all-free-downloads, pixabay, pexels etc?

ILLUMINATI!