Hi, and Welcome!
Every winter [December IIRC], Continentals [not the trained ones], Militia and Provincial troops have a probability to 'go home'.
To understand the 'nuts and bolts', see:
http://www.ageod-forum.com/showthread.php?t=8417&highlight=AutoRaise
Also, if you had a 'Revolt' where local militia captured an enemy city, those troops 'dissolve' in a few turns....
edit: Rafiki types faster than I
