what causes sibilance?

other than bad recordings, that is. I recently bought a VPI 17F record cleaner and began cleaning records with Disc Doctor Miracle (hand wash & rinse, then rinse with vacuum) Amazing difference! Using an EAR 834P phono pre, AR LS-15 pre, Mesa Baron w/Siemans EL34's, Infinity Compositions speakers, mostly MIT cables, and never heard a trace of sibilance before. Upon cleaning a MoFi version of the Dead's American Beauty, all of a sudden there is voacal sibilance. What's going on?
S, sh, z etc.

Some vocal artists are better than others at avoiding this. Microphones pick it up. In normal situations this is attentuated (high frequencies attenuate faster than low) so you don't notice sibilance.

A sibilant sound will make it feel like that artist is standing closer to you. It can get annoying.

A good system with a clean source will pick sibilance up more than a dirty source. Imagine your LP has dust and grime from being years old. This debris will not affect low frequencies but will dampen high frequencies. After a while you get use to how it sounds and the tone ddown high frequencies sound correct to your ears. When you clean the LP and remove dust and dirt and debris then asll of sudden you hear high frequencies again... all that original sibilance in the recording comes back.

A 3db notch filter centered 7 Khz will help if you don't like the new sound... if you have a PEQ you can play with the Q unitil you get it right. A cleaned LP is probably closer to how the LP sounded when new anyway - so give it some time and you may get used to it.
Several things can cause excessive sibilance.

Another source: I get increased, spitty sibilance when the VTA is set too high.

I have a Technics SL1210, so I can play with VTA very easily.
Clearly, you may receive many different answers/opinions on what your problem may be. So my opinion will only be one of many!

While the Disc Doctor Miracle is supposed to be a top quality cleaning product, it is also, from what I understand, notorious for needing LOTS & LOTS of rinsing. I am only venturing a guess here that perhaps you did not rinse enough on that particular record and that residue from the Disc Doctor may be contributing to the sibilance you are hearing. If you did not have sibilance issues before, a re-clean of that record followed by heavy rinsing, perhaps 2 or 3 times with high purity water would be the first thing I would do to see if that solves the problem.
only on that recording? If that is the case, I'd say it's the recording that is sibilant.
If you have the cd version, it would be interesting to compare as far as sibilance, although it may not mean much in the end since the record is dated to 1978...obviously CD will be mastered differently and much later, but still an interesting test. If it's not the recording, reclean with other fluid may be...and if that doesn't help, check the VTA.

Lots of reasons for sibilance. It's just that some systems will be more prone to it than others. Depends on the overall balance of the system, components themselves, type of tweeters(metal domes are more prone to sibilance than say silk), etc...
thanks for the reponses. will try vta adjustment and re-rinsing. The cd version doesn't have any sibilance to speak of. It's probably not just this record, I only recently started cleaning them adequately.
Don't forget to keep the stylus clean. That was my first experience with spitty sibilance 35 years ago--a big ball of fuzz stuck to the needle. Use a Mr. Clean Magic Eraser, and then a soft brush to remove the Magic Eraser residue.