Answer:
Doing something a person in that position shouldn't be doing, even if it's maybe not strictly illegal.
A small kid might get away with being visibly(and noisily) upset if their favourite candy has sold out, but as an adult you're expected to be able to handle that kind of events. So an adult getting all worked up about something like that could be an example of conduct unbecoming an adult.
Most military organisations have their own laws and regulations.
In most armed services this would be a 'catch all' category for officers not covered by other regulations. A sort of 'Catch 22', if the authorities have no specific charges this is the one they would use.