Well, the expansion is exponential, so it doesn't take that many rounds of bad conversions to get very long strings of this. Any kind of editing or transport process that might be applied multiple times and has mismatched input and output encodings could be the cause; I vaguely remember multiple rounds of “edit this thing I just posted” doing something similar in the 1990s when encoding problems were more the norm, but I don't know what the Internet Archive or users' browsers might have been doing in these particular cases. Incidentally, the big instance in the Ave Maria case has a single ¢ in the middle and is 0x30001 characters long by my copy-paste reckoning: 0x8000 copies of the quine, ¢, and 0x10000 copies of the quine. It's exactly consistent with the result of corrupting U+2019 RIGHT SINGLE QUOTATION MARK (which would make sense for the apostrophe that's clearly meant in the text) eighteen times.