The first thing a CM may say is "it's demagnetized". It isn't necessarily true, but its a easy answer.
X-ray machines and metal detectors are generally not a problem. To really corrupt the data (it isn't really demagnetized , but "scrambled"), it would have to come in very close proximity with a strong magnetic field. If you took them out and rubbed them all over the metal detector, then yeah, you've probably messed them up.
They ARE more susceptible to corruption than a credit card - because a credit card isn't meant to be updatable.
They are pretty much on par with the "hard plastic" cards in that regard, but people do feel "safer" with the plastic. CMs have reported in the past that the plastic cards actually often gave them more problems.
In the past couple weeks, with the record low temps, I wouldn't be surprised if there were two non-magnetic issues at work - cold readers and cold tickets. With the cold readers condensation can build on them, and moisture is a problem a bit more with the Tyvek tickets.
I actually had problems more with the finger scan than my ticket. No matter what, it seemed to have trouble reading my finger (YES, they do actually do finger scans!) and it would take two or three attempts (again, possibly due to the cold. My AP worked perfectly fine, although one of the kids MYW tickets occasionally needed a second scan at the turnstiles. No problems at all at the FP kiosks though.
Oh, that's right, they are NOT paper, but Tyvek, which is in fact a form of plastic. They are waterproof - sort of. Liquid water can't pass through them, but water vapor can - anecdotally "rubbing" the ticket to warm it up can sometimes (not always) help (it helps release the moisture, but it generally takes a lot of work - easier to replace it) - just don't rub it on your smartphone (there is a magnetometer in most fancy phones) or your magnetic clasp on your purse, or anything with a magnetic/EM field (most cell phones, really) etc.
You generally can't ask for a plastic ticket. If you order MYW tickets direct from Disney, they are usually plastic - probably because they stand up better in the mail. You might get plastic from other sources.
UT sends Tyvek, and tickets bought at the parks are Tyvek. Tickets obtained at a resort separate from the KTTW card are usually plastic, only because they only have the plastic card printers - and they will be on KTTW stock.
Keep in mind, the GREAT MAJORITY of tickets that aren't on KTTW cards are Tyvek, so its no surprise there are more reports of problems. and even if the error rate was higher than for plastic, the per-ticket cost is a fraction of plastic.
Even Annual Passes and the ilk are Tyvek, except the Premier Passports (which is why you can only get a PP in certain places - they don't have the plastic card printers everywhere). And APs get used a LOT.
I keep mine in a hard plastic waterproof case (I got it from the Beach Club). I keep everything in it facing the same way so the magstrips never actually touch (in theory, if two magstrips touch and rub past each other, it could have an effect). The case keeps them sufficiently away from all but the strongest magnetic fields.
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