Is a backdoor draft possible?

No such thing! Read your contract it's 8 years for everyone!!! Unless you serve 8 years you have remaining commitment and if you are called to fulfill that commitment it is NOT a "back door draft".

In January of 1991 I got a "draft" notice. As a medic I was ordered back to Active Duty at Fort Lewis WA. As it turned out my National Guard unit had neglected to update my status and DA thought I was in the IRR, so I never reported. One of my buddies from Korea was not as lucky and ended up in Fort Campbell for a couple of months of babysitting a telephone that rang exactly once a day. They had recalled hundreds of medics to Active Duty to await the massive influx of wounded soldiers from Operation Desert Shield\Storm. It never happened so he spent a couple of months literally doing nothing, they had a duty roster of who had to pull a phone watch during business hours, the phone rang once a day for accountability reasons. It did give him an opportunity to come visit me in Louisville KY and meet my family, but otherwise it nearly bankrupted him as we was a small business owner and had to rely on his staff (2 friends) and his silent partner to keep the shop open. It survived, barely and he was able to close it later without too much loss.
So if you wish to call the involuntary recall & stop/loss a backdoor draft the answer would be yes, but I thought of it as the contract we all signed as volunteers.

Even with the shrinking forces we still have thousands of young Americans enlisting and commissioning. I don't think the would do something like a backdoor draft.