Need clarifications.

Salaam my dear brothers and sisters.

Just awhile ago, i was doing my dhuhr prayers at the mosque in congregation. During the last sujud, the imam gave the takbeer and when i rose up for the tashahhud, i saw him rising up to stand(it was already the last rakaat) and some of the people behind him made some sounds to indicate to him. He immediately went back down and went into the position of tashahhud. So i did my tashahhud as per normal when suddenly the imam said the takbeer and went into prostation, went back up, and again into prostation and back(everyone followed his movements). And immediately after gave the salaam.

Can anyone explain this? Why did he do the sujud all over again, afterwhich immediately give salaam?

Sorry for the long explanation. I hope you can understand what i am trying to bringforth.

Thank you all in advance.

As you said, the Imam made mistake. So, after finished salah he did "sujud sahwi".

He did "sujud sahwi" because he realized he made mistake in salah.

Thank you for sharing this experience bro, I have read about the "Prostration of forgetfulness" but have never actually known how it is performed.

Thank you brother ardianto and Hulk for that. Really appreciate it. I have heard of of sujud sahwi but never really knew what it actually was.