Malayalam linguistics question

This is a question in linguistics. Or something. I am hoping that the 3.5 readers of this blog will have some input regarding this.

There I was, on Wikipedia, as usual. On a slightly related tangent, let me just say that Wikipedia provides me an excellent outlet for all this useless knowledge that I have stored up in my head. I mean, there are people there who find comparative linguistics as interesting as I do. Also, where else can I use this absolute gem that I have:

Romulan Disruptor fire is characterized by a high level of residual antiprotons.

On Wikipedia, that is! All those hours of watching Star Trek: The Next Generation totally paid off! Woohoo! (+5 Nerd points. -5 Cool points). Well, anyway, onto my linguistics question. So I was checking out the Malayalam language article on Wikipedia, and noticed this:

A very few people whose Sanskrit names end in “a” are given the plural suffix “-r” rather than normal “n” because they are revered, but this is extremely inconsistent – for example, Shankaracharya becomes Shankaracharyar but Agastya becomes Agastyan.

This sentence has since been removed, because it has no citation. I know the statement to be true, since I am a native speaker of the language, but I have not been able to find any linguistic basis as to why this is so. All I know is that it is extremely inconsistent. One other example I have is Bhishma, which becomes Bishmar in Malayalam. What is even more interesting is that the -r ending on nouns is usually used to pluralize common-gendered nouns. For example, the Malayalam word manushyan (man) is manushyar (men) in the plural. The -r ending might also be a remnant from Tamil, where it is more common.

So anybody have any information (citations would be nice) regarding this?