Hmm, according to the wiki article (elderscrolls.wikia.com/wiki/Lydia) she has no magic abilities, but is able to use spell scrolls. If she is carrying one she might be using it, but since this has been happening for quite some time I'm sure she would have run out by now. Since she has no enchanted equipment, this is quite possibly a bug.

Not do dig up an ancient question from the dead or anything, but perhaps she had an ability that imbued her equipped weapon with some sort of magic?

Yes, and if there is a limit, it may be something like 256 or 64 or something else arbitrary to stop the engine from creating too many due to an unknown bug and crashing the game. It's not likely that there is a limit on how many you can spawn, you just need to be fast enough and crit often enough.

I did some testing with socketed items of different kinds and got some interesting results that seem to give creedence to the idea that rubies are either broken or by design alter slower weapons differently to faster ones. i.imgur.com/1HFjG.jpg