An article on Big Bang not enlisting together brought up TVXQ as a contrast:

It isn’t hard to find a group whose members enlisted in the same period in order to minimise their (group) hiatus. TVXQ’s U-Know Yunho and Choikang Changmin each enlisted last July and November to fulfill their military duties while shortening their hiatus. Although Changmin was born in 1988 is two years younger than Yunho, and could have delayed (his enlistment) until 2017, he enlisted early for the sake of team activities.

{t/n: rest about another group omitted}

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Credits:

Sports DongA,
Translated by @snxy,
Shared by TVXQ! Express