I agree with you, your pictures are terrible

I can't see if it is the same types of carpet I've seen this happen to, but have experienced this problem myself.
It's always been with polyprop carpet. Someone correct me if I'm wrong but I think polyprop is "solution" dyed, i.e. the liquid polyprop is mixed with a liquid dye to obtain the right colour when it sets.
The best explanation I could find was the different dyes have different effects on the material properties of the carpet fibre - in this case the red is making it less durable, and it is simply breaking up and wearing away at a noticeably faster rate than the other colour in the carpet.
Doesn't have to be direct sunlight, could be another cause. If it's in an entrance way then the salt option looks plausible, partly due to alkalinity and partly due to extreme abrasion/ wear underfoot.
I too have been horrified by the red fuzz collecting round my rotary brush, on my wand, etc etc... but in the worst case of this happening it was my first! I didn't know what it was and didn't really see it before starting the clean. Luckily, with a professional explanation, the customer was satisfied that I hadn't done anything wrong as such. I've seen a few other minor instances of it since, and always pre-qualified... it's just part of the learning curve
