what causes sibilance?

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.