As the Nine-Tails finally emerged from theseal, Tobi used his Sharingan to gain control of thetailed beast and commands it to destroy Konoha. My analysis:The main reason however is notably clear.Tobi/Madara took advantage of the weakened sealbecause of Kushinas child birth and to take control over theKyuubi.

One may also ask, how old was obito when he killed Minato? 14

Also know, why did Kurama attack Konoha?

Kurama being controlled by Madara to fightHashirama. After Madara Uchiha defected from Konoha, he usedhis Sharingan to control and use Kurama to help him fightagainst Hashirama Senju, the First Hokage, in order to exact hisrevenge.

Why did obito help Itachi?

Tobi aka Obito was just doing his mission whatMadara gave to him to do , Since Obito didnt really caredabout their village or his clan anymore because of Rins death, Hehates everybody for not helping Rin.