For outdoor cats, the warmer weather would seem to be a trigger for them to shed fur, but for indoor cats, who live in constant room temperature, what triggers them to shed in the spring? How do their bodies “know” it’s time to shed?
Differences in humidity?
I haven’t actually noticed a big seasonal difference in shedding in my indoor cats, unlike the dogs who spend plenty of time doing stuff outdoors. In my experience at least, shedding isn’t predetermined like bird migration; it’s quite definitely related to ambient temperature and conditions.
There’s plenty of non-seasonal reasons an animal will start losing coat - diet, parasites, immune deficiencies, stress.
The instinct to annoy.
In horses, it’s triggered by the increase in daylight that comes as Spring arrives. Also causes mares to come into heat, so breeders will sometimes install special lights in their stalls to mimic the increased hours of light of springtime.
Possibly cats also are triggered by hours of daylight? This would affect both indoor & outdoor cats.
My vet tells me that indoor cats just shed year-round. I don’t know how authoritative it is, but this link says that shedding is linked to the amount of sunlight a cat gets. Indoor cats are exposed to more artificial light, so they are less likely to shed seasonally.
My indoor cats shed all the time.
Most seasonal cycles in animals aren’t triggered by heat/weather, but by daylight cycles.
The same typically goes for plants, as well.
The trigger for a cat’s shedding is the presence of clothing or furnishings of a contrasting color. It has been demonstrated that a white cat can lose up to one-third of it’s fur when placed in a room of people wearing black. 
Huh, and I thought it was allergies.