Lets say I create a new stereo bus, which gets named by Sonar to A. I then go and do all my work using it creating sends etc in the various tracks. Then decide to add another stereo bus, why does Sonar automatically rename the existing bus to B and insert a new "A"?

I always thought Sonar changed the newly added bus to "B".   I have never just left the bus name to "A" or "B", I've always changed the bus name to what I wanted.

Thats not what I remember happening. If I insert a new bus it names it the next in sequence. However, I usually rename my buses so I don't see this often anyway

Just tested and that doesn't happen here: the first bus is Bus A, the second is Bus B.

hmmm, thanks I'll go back and re-do it and see.

It depends where you insert the bus ... before A will cause the rename of the original A to B, but after A will leave A unchanged and create a B.

What Steve just said.
It's totally depending on where you click to insert the buss. I just tried this after reading this thread. 
If I click in the blank space below the existing busses I get the correct sequence. A small 2 choice dialog that opens. 
If I have the mouse on Buss A, a larger dialog opens and inserting a new buss puts the new buss above A, names it A and the original A becomes B.