Sony Trinitron color fading

[jt01]

There's something I've always wondered about relating to my history with Sony Trinitrons. I bought a 17 inch Trinitron in 1978 - it was great until the color started fading in the mid 80s. Replaced that in 1985, passing the 78 model on to friends, with whom it shortly faded completely to b+w. The 19 inch 1985 Trinitron set was fine until the early 90s, when again the color started fading. Replaced it in 1992 with a used 27 inch Sony, vintage 1989, I think. This lasted a bit longer and the color didn't fade, but I replaced it for other reasons in 1999 with a Panasonic. My question is - Trinitrons were excellent TV sets, but was there something about them that caused color fading after about 7 years? As you can see from the above, it happened to me twice. Sorry I don't have model numbers, etc. but they are all long gone.