～次第です — The Reason Being…

急な用事ができ、欠席させていただく次第です。

Kyuu na youji ga deki, kesseki sasete itadaku shidai desu.

An urgent matter has come up, and so I will be absent — that is the situation.

次第です formally explains the reason for the absence

そういうわけで、お願いに参った次第です。

Sou iu wake de, onegai ni maitta shidai desu.

For that reason, I have come to make this request.

参った (humble 'came') + 次第です explains why the speaker has come

皆様のご協力のおかげで、無事に完成した次第です。

Minasama no gokyouryoku no okage de, buji ni kansei shita shidai desu.

Thanks to everyone's cooperation, it has been completed without incident — such is the case.

完成した + 次第です gives a polite account of the outcome