You can find a more technical explanation if you try a search, but broadly: if a unit is already destroyed and in the queue to reconstitute when its parent formation is withdrawn - in this case by a surrender Event - the unit will still reappear. It does mean that in EA you occasionally get several units from a surrendered country reappearing, e.g. French and Belgians after the fall of western Europe. Nothing to be done, if you disband them they simply reenter the queue and pop up again like vampires. :bite:
We've a House Rule that they constitute die-hard remnants determined to fight on, exiles reforming abroad, large bands of guerillas, etc., and leave them in play. Happens to both sides so probably evens out if you go the distance, but it does mean that the Allies sometimes get significantly larger Free French earlier than they did. They're usually out of supply and pretty weak, so they can be chased down and killed by flak, HQs and security units. The biggest problem is with Soviet western front units scheduled to reappear in - say - Kiev, but you simply have to suspend belief and accept them as large, 'early' partisan bands. Big picture scenario, ahem.
If we'd had enough Events, a second withdrawal about half a dozen turns after the first surrender would probably sweep most of them up.
