Example 1. Really didn't want to be there fighting for that country.
Such as the Neapolitan regiments in Spain, many of whose men were delivered in chains. They deserted, got captured again back in Calabria, transported to Spain, deserted, got cap....

Example 2. At major battles the nearby inns were apparently frequently filled to capacity, and would only empty several days later. Combined with troops temporarily captured this meant that nations who took a roll call the day after a battle had very high 'absent' numbers (Nederlanders 1815) while those who waited a week or two for them to drift back in (British 1815) had very low numbers of absent.

Example 3. Supplies ran out, and it was 'long distance forage' or starve.

Example 4. When it seemed easier to be a full time brigand than a part time one.
.........

Jeff Lewis