These are Google Books' "ngrams". You can view the frequency of some particular expression over time. I was surprised by the pattern of " none of them was" vs. "none of them were". " Was" became more popular for a few decades, but "were" is making a comeback!
I wonder if this has to do with grammar teaching in schools. We have it beaten into our heads that "none" is singular, so you better use a singular verb or else!! But it's well known that people have a tendency to use a verb that agrees with the closest noun in the subject, regardless of whether that noun is the head of the phrase or not (this is called "agreement attraction" in the psycholinguistic literature).
Agreement attraction isn't a bad thing - because we hear people use it and use it ourselves all the time, it doesn't interfere with our ability to communicate with or understand someone else. In fact, using infrequent forms like the "correct" grammatical form actually results in slower processing. If we want communication to be efficient there's no time to waste using the forms that your holier-than-thou middle school English teacher taught you.
So look out, prescriptivists - it looks like agreement attraction is coming back whether you like it or not!