Search Engines use algorithms to determine organic – remember, organic links are the ones that appear beneath all those Ads in a search result (we call those SERPs by the way – Search Engine Results Pages) – results when you search for something.
The algorithm at Google is a closely guarded secret, it’s what gives them their edge. I doubt any one person knows every single aspect of their algorithm, it’s too risky & I’m also sure there’s more than 1 algorithm at play for organic search. Every now & then Google updates their ‘core’ algorithm and it’s this that shakes up the search results so much – and when people in my industry scream, cover their eyes & hope it goes away soon. OK, some do…
It’s thought there are around 200 aspects of this ‘core’ algorithm and Google tweak these – and their weighting within – every single day. Mostly, these have little to no effect but every now and then it causes a big shake-up of things.
In the old days, these big changes had animal-related names which you may have heard of such as Penguin (links) and Panda (content), but these have switched lately to terms like Medic and Fred (Fred is a bit of an in-joke). Don’t chase the algorithm, just do SEO well.

Here’s a great guide to the main algo updates from Google via those clever people at Yoast – enjoy!

Finally, those clever people over at SEMrush have a free to use “Sensor” to see what they feel Google is up to any given day. This is a real time analysis of the current state.