I don't think there is a single brand that hasn't had this problem and it seems to be very random. I have come across a lot of watches that had something (?) on the inside of the crystals from just about every brand a lot of people I know that bought them in big department stores and mall jewelers. Probably because these people bring me these watches to work on are not high rollers all they can afford is brands like Seiko, Citizen, Bulova, Movado etc. My friends with the high end watches bring them to a professional to service and truthfully I wouldn't touch a high end watch unless it was my own. One girl I know brought me her Raymond Weil with the fogged over underside of the crystal. I don't know what it is but it does clean off very easy. I don't know exactly what causes it but I would believe what Rod says.
Recently I got an old Citizen that a friend said thought needed a new crystal. When I saw it I saw a pattern to the marks on the inside of the crystal. I put on a loupe and could easily see it was a finger print! Good enough to get a good print from. I don't know if it happened in the factory or maybe if it was serviced in the past but it was there. It was probably invisible but as time when by and either fine dust got in, moisture or the oil evaporating inside it worked like finger print dust and made it very prominent.
As far as it happening to me, I have tons of watches and I have not noticed it on a single one. YET! It could be environment, Because I collect guitars I keep the humidity at the 45-55% range year round.
Time is an equal opportunity employer. Each human being has exactly the same number of hours and minutes every day. Rich people can't buy more hours. Scientists can't invent new minutes. And you can't save time to spend it on another day. Even so, time is amazingly fair and forgiving. No matter how much time you've wasted in the past, you still have an entire tomorrow.