Why Do All Muslims in Tamil Nadu Speak Urdu?

Yesterday I was reading my motorcycle its last rites at the mechanic shop a couple hundred meters from my front door. For the last year I've had trouble talking to the man because of an intractable language barrier. But recently things have begun to change. He hired a Muslim assistant from nearby who speaks urdu and since then I've been able to explain exactly how dilapidated by bike really is.

When I asked the mechanic why he spoke Urdu he looked at me as if I had just asked the stupidest question he had ever heard and said "Mai Musalmaan hoon", as if that explained everything.

So call me crazy, but since when did being a Muslim imply that you definitely know Urdu? I've traveled all around India and found that most Muslims do speak it--even in some rural outposts in the middle of nowhere--but it doesn't really make sense to me. Most of these people have never traveled North and must only speak Urdu with their family. Many of their families have lived in Tamil Nadu for dozens of generations. So how did Urdu spread so widely down here?

I'm not complaining. The fact that most Muslims understand Urdu is a huge boon to me and allows me to travel all over the country with the understanding that at least I will be able to speak to someone in every village and t own. I just would like to know how it happened. Was it Mughal influence? Were there huge migrations of Muslims from the North to south? Were there mass conversions/language lessons? I refuse to believe that every muslim in south india is a recent north Indian transplant, and yet I can't understand how the whole community can keep the language intact over the generations.