Chronovore'd!

In the Dresden Files version of reality, magic disrupts modern technology. This is something wizards take for understood, but none of them seem to know why this is the case. It just is. Older technology works better with magic to a large degree, and when you get a century or so back, things are just fine.

Why?

The books work with physics when it can, so why is magic incompatible with tech? Electrochemical batteries (of a slightly less refined sort) have been around for centuries, and hydrodynamically-powered doors (of a quite remarkable size) work. Wizards can use lightning just fine, if they're careful.

I only see (at the moment) a couple possibilities as to why tech is so allergic to magic.
The first is that magic around a wizard can only handle reality that the wizard understands, and tends to break the rest of it. This would explain why few wizards would bother to try and get an electrical engineering degree: it wouldn't work for them until they already understood it correctly, and their experimenting with it would be fruitless until they knew it already.

I don't see the first case working very well, otherwise more young or uneducated magic-using types in the modern world would be flying around or playing out roles from superhero comic books.

The other possibility I see: some power or group that deals with the world has influenced or designed the last century or so's worth of mortal advances so that it will break down in the face of magic. As mankind becomes more dependant on the modern technology, and the more advanced it is, the sooner it will break into uselessness when some magical predator from the Nevernever comes to play with the nonmagic humans. "We're not scared of you, we've got some new toys to shoot you monsters with! ...why is none of this working? These batteries can't be dead, they're new! At least the automated turrets are working, right? It can't BSOD, it was running Linux!"

From fairly-defended humans back to snacks-in-a-can in a century or two.

Jim, for humans of your world to have any hope, better get some of your magic practitioners some science books and get them working on tech that'll still work in the face of magic. It's that or all the nonmagic humans will get killed off and eventually slowly replenished with a full race of magically-enabled humans. You know, if there's any left, since they seem to be being targeted first.

Someone has gifted me the series FLCL.
I was fairly hyper about it for a while, until it turned out that not a single DVD player in my residence will play any of it. Won't even recognize the disc as existing. Annoying, but since I'm almost exactly where I started, I just shrug and ignore it.

Well, how about that. 12 hours after my cat died, my computer monitor did as well. If all goes well, I'll have a new one by tuesday. If not, it may be next friday.