We know phenomena is a plural whose singular form is phenomenon. However, I have seen frequent of use of phenomena itself as singular, as in 'this is a phenomena …', 'this phenomena is …', etc.
This Google Ngram shows such use may also have been present in literature, 'this phenomena' being more frequent.
How do we explain this discrepancy and suggest that the usage is wrong?
[Edit 1]: Will usage eventually redefine grammar in such cases as 'phenomena' and 'criteria' then? (inspired by @Barrie England 's comment.)