If you use a hammer to start mining, you go into battle mode.
Intentional? Yes, but it also allows you to use in-battle skills, such as Mantra and Beef Up. More of a balance issue than a bug.
In short: Use a hammer on a mountainside, Mantra to full HP and MP, save on potions.
Um...you can enter battle mode whenever you want...it's not like it's hard.
Oh My God This Bug Is going to Break The Game Gaku Must Fix It Immediately !
You go into battle mode when mining/chopping wood since it looks rather odd when you're trying to mine/chop in OW.
Gaku: not a bug