I don't think it's down to the fitting of the carpets per se, but definitely if preventive measures are taken during fitting the problem can be greatly reduced. The best safeguard before fitting is to seal the gaps between floor and skirting boards, then lay lining paper down on the floor and tape the joints. The underlay should have the joints taped up too.
I've heard quite a few different theories as to the cause of filtration soiling, ranging through atmospheric situations like John says above, static electricity built up in the carpet making the dirt bond to the fibres, the type of fibre itself, how often the customer vacuums, suspended floors, blah blah..
I think it must be a combination of several things, in my opinion. I know that in some old houses with suspended floors they can be real bad. In these places, the lines aren't just confined to the edges, they clearly mark out the gaps in the floor boards and underlay. It would be logical in these cases to presume that there's a lot of crud underneath due to the age of the house, and the air flows upwards bringing the particles with it.
But could it actually be going downwards? The particles have a lot of work to do to get through the gaps, then penetrate the underlay, then the backing of the carpet and finally get to the top! I don't know how small the particles typically are, perhaps it's possible.
Around the edges of a room with a concrete floor, what's going on there? Air could hit the wall and cool, descending down to the floor and carrying the particles with it. Air could be travelling around the room in a certain direction at floor level.
Airing cupboard doors are the worst when there's a hot water cylinder inside. The hot air escapes quite rapidly from the top, dragging in air under the door. Had one today that was really bad. The same customer had me on my hands and knees spotting marks the size of a ladybird's wrist watch
Gotta be the most neurotic male customer I've ever worked for!
Anyway, as for removal of draught marks. Basically, every supplier will claim that their spotters are wonderful for removing draught marks, I've tried a few and they're all as crap as each other. Filter out is just a diluted Ultrapac renovate, which I've tried at several dilutions, hot and cold, short dwell, long dwell, plenty of agitation... then chucked it on the shelf with the others.
The best results will come on a carpet fibre that's naturally resistant to them, rather than from a certain product, so there is always an opportunity for apparent heroism with little effort
It depends how long it's been happening too, if you get to a wool carpet after 5 years of draught marks building up, forget it, you ain't gonna come out the hero! I think polypropylene must stand the most chance for having the marks removed, or at least faded considerably. Hard-wearing lop piles such as the stuff you get in rentals is probably where I've had the most profound results. Ironic really, considering it's probably the type of job where such a "miracle" would be least appreciated!