Military behind lines can be pretty important because of delays in them receiving notices and their ability to do anything about it. Pretty much every legal notice gets a timeframe before things occur. Usually something like a month is sufficient, but for someone in military service, who have arguably the best excuse to not be able to do regular things for a whole month, the usual polite way to go about things is to wait for them to return home, and historically, courts have thrown out cases for people who didn’t do that, because it wasn’t a reasonable amount of notice.

No idea about the legalities of this particular case, but it is extra rude, getting a court date and losing by default for not showing up when you didn’t show up because another part of government mandated you be on a battlefield.