Can be a lot of things. First it's very frequent to find slightly off center panning on a track for a bunch of reasons. So you should start by listening to something in mono and figure out if that feels spot on center, or not. If it is then most likely the tracks are just made that way and you happen to be a little to sensitive about it.
Then if it's not full center, you turn your headphone around and see if that changes the direction of the imbalance.
If the sound remains on the same side, it's something about you. Some asymmetry on your face/ears(we all have some amount of that, the question is just to figure out if it causes a significantly audible change). Or maybe you just spend most of your day looking slightly on one side(more typical than we think). Whatever the cause, the end result is that your brain expects that asymmetry and compensates for it so that what you hear aligns with where you see the sound source. When you listen with your headphone and bypass the physical cause for the imbalance, you end up with your brain still applying the compensation and now thinking there is an imbalance in the other direction. not much to do about it beside adding that imbalnce on the headphone playback to put things like they usually are(as in, without headphones).
If the cause is a habit of looking sideways, it's possible that instead of just a gain imbalance, what you need is to add a tiny delay on one side. I remember someone in a similar situation reaching such conclusion after many experiments.
On the other hand, if the sound changes side when you turn around the headphone, then the imbalance is on your rig. Could be the headphone, in winch case the imbalance could even be only at some frequencies instead of a global imbalance(that is in fact the case for almost all headphones, we typically don't notice or pay attention, but it's there. all you can do is try to measure and compensate that imbalance(Sonarworks offers to help with that for $$$), or simply to get a headphone with a better matching.
It could also be your amplifier, maybe some consistent imbalance, maybe something that only becomes significant at some position on the volume knob(depends on the type of volume control). DACs usually have very closely matched channels so I wouldn't worry about it before confirming that it's not any of the other possibilities.
