Board Thread:Naruto Discussions/@comment-32624156-20170721153715/@comment-1512913-20170730082718

Hago made the Tailed Beasts before his death, obviously, having gotten inspired by Asura. Hago also made Asura his successor before he died, which means Asura must have known about the Tailed Beasts and they knew him. Most likely, the Tailed Beasts had instruction to follow Asura's lead from Hagoromo.

Maybe when they saw that Asura's brother Indra wants to kill him, they willingly shared their chakras with Asura, in the same fashion as with Naruto, through fist bump.