Lots of things can cause loss of liquid. If you use a waterbath method, not covering the jars with water will cause a loss of liquid. If you are pressure canning, dropping pressure too quickly or wide ranges of pressure during canning. Not getting the air bubbles out or not having the rings tight enough can cause that. I'm probably forgetting a few possible causes.
With peaches, you were probably using the waterbath method. Since it was by batch and not individual jars that you saw the problem (I think) I'd question if the water was 2" over the top of the jars. If it were by individual jar, iId think most likely air bubbles. But it could be something else entirely.
With cold pack, it is not unusual for the peaches to float up and leave a lot of liquid at the bottom, but you should not lose liquid.