If the ball is sliding across the cloth it picks up topspin, but no way to tell if it's the cloth or if your tip is rising.
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This prevents it from picking up any friction from the cloth.
From my work in slow motion, I can add a couple of minor points here.
Although the airborne cue ball doesn't pick up forward spin while it's actually off the cloth, it does pick up a good amount of spin, more than you might think, when it lands (assuming it lands before it hits the rack).
Also, if the CB does bounce a few inches before it hits the head ball, it will make contact with the rack while on an upward trajectory, and will actually pick up forward spin from hitting the head ball "off vertical center." As you know, in a normal cut, the CB picks up some sidespin after contact. This is the same thing, but in the vertical plane. The bottom of the CB hits the top of the head ball, thus inducing some forward spin on the CB.
That's why a well-executed "squat" break with no initial spin on the CB will actually result in the CB rebounding backwards off the rack with a little forward spin, then landing and "using" that forward spin to halt, right in the middle of the table.
You can see what I mean in this video, at 46 seconds:
http://youtube.com/watch?v=KdWnR1wFsc0
In this example, I had a little bit of backspin on the CB before rack contact, but the cloth landing and "elevated" 1-ball contact result in the CB bouncing up and back with almost no spin. Of course, when it lands on the cloth (after rack contact), it picks up natural roll in the new direction of travel (back towards the head string).
Fascinating stuff in slow motion. This is something I'm working on for my next video.
The only way I know of to truly verify what spin is initially on the CB (before it hits the rack) is with a high speed (slow motion) camera. "Reading" the result of the break doesn't quite tell the whole story.
Anyway, to the OP, based on your description, I would have to agree that you are probably inadvertently putting top spin on the ball when your break behaves that way.
-Blake