Why was the AT&T assembly syntax designed this way?

The AT&T assembly syntax, often called the GAS syntax, keeps surprising me. For example, its parameter order:

mov $100,%eax /* Destination after source */

This just seems so counter-intuitive! Nearly all programming languages (and the good old mathematical notation) go the other way around: destination before source, like this:

int a = 100;   /* Destination before source (C-style) */
a := 100       // Destination before source (Pascal-style)
mov eax,100    ;  Destination before source (Intel assembly syntax)

There are other points, for example, sigils… The %-s before every register and the $ before every constant literal (except for those used as the fourth operand in the very strange-looking effective addressing syntax) mightily clutter up the code, when it would have been trivial to avoid them. What is, or was, their advantage?