Exactly, the trigger for me, who chooses to use winter and summer tyres, is the probability of temperatures dropping to or below +4C - with the hopefully added bonus of winter tyres coping better with snow when it arrives.
In my mind, it was not always so, in my motoring lifetime, quite so clear as it is now, "tyres" have modified to give longer service, be quieter and produce less in the way of "lost energy", ie be more eco, this works/worked very well for the majority of, in our case, UK drivers, but weather conditions seem to have changed bringing more rain to the typically drier areas in winter, and along with the changed priorities concerning tyre design leaves me thinking that normal tyres have been changed to being more Summer tyres, which can and does cause some issues.
The sudden change to using winter tyres on my wife's 2002 VW Polo was prompted by her making comments about the grip or lack of it that she was experiencing with Michelin "something" - I've forgotten the name, but on checking up on the Michelin website, they seem to have been dropped from their range of tyres. These tyres, I later discovered, while being a very good warm weather tyre, were almost dangerous when used in Northern UK in winter time. This was confirmed to me when I needed to replace a pair, the tyre fitter in our local Costco said "you are not planning on using them in winter are you?" I confirmed that I had already worked that out and had winter tyres, Alpins on that car, I was just getting 2 tyres replaced while the wheels were not on that car.
I was running Michelin Primacy on my 2000 VW Passat 4Motion at that time and continued to use them without any issues even in winter, then maybe a couple of years later, changing weather meant that I had trouble driving down my driveway while staying in control, reversing up and out was still okay though, so I quickly searched out a set of Michelin Alpins, but none of the usual outlets had any stock, which did happen a lot back then in maybe 2006 or 2008, so ebay came to the rescue, luckily some silly person had tried to fit that size of winter tyre to an Audi A3 and ended up with fouling, so I bought them, and binned the Michelin Primacy, then set out looking for a set of Summer alloy wheels and fitted them with maybe Dunlops in ZR rating, so win - win.