You are assigned to a certain hub. However, you may change to a different hub after a given time (usually when you switch aircraft) or a hub may close and you will be displaced. For example, my company started with 1 hub (Washington DC) and then opened a 2nd in Chicago (both fly the Canadair Regional Jet for United Express). We then in quick succession got a contract with Delta opening a hub in New York, then Boston, then Cincinatti - and then just closed the New York hub. These bases fly the Dornier Regional Jet. If you switch jets (due to location, upgrade availability from F.O. to Captain, or want to make more money (the CRJ pays about $10 more per hour) you end up switching bases. Thus with all of this moving around, it makes more sense to live at your military reserve base because you will probably make a substantial portion of your income there - and you will likely stay in that unit until you retire.
Your airline schedule changes monthly. You bid for a certain schedule (say for 2,3,or 4 day long trips) and repeat this for the entire month. Next month you have to rebid for a schedule. Since the selection is based on seniority - and the routes flown by the airline change monthly, there is no guarantee you will get the same monthly schedule.
The 10 year mark is the point where most are making the decision: stay in the Corps or do something else. Some do the airlines (and join a hometown reserve unit regardless of service), some go Customs or FBI, some do an interservice transfer, some go to graduate school. The reasons are varied, but most often its for a change of scenery or to get setup for your career. If your goal is to fly for a major airline, then you need fixed wing time which means a transfer to the Air Force or getting out and going to the regional airlines. The FBI and Customs have a mandated requirement that you have to be hired before you turn 37 years old and since they are not always hiring, one has to get in while they can. Finally, if you are going to grad school, you need to get in, get your master degree, and then get started with a company while you are in your 30's. So basically, you start making decisions when you are in you early to mid 30's (around the 10 year mark in the Corps.)
Even though you eventually move on, if you did 2 years or 20, its still "Once a Marine, Always a Marine" Semper Fi.