Vamana Avatara, It can be that Maharaja Bali was bent upon taking the position of Indra. Indira has a life cycle during which time he remains the king of devas. By trying to out him, AKing Bali was going against the rule of nature and was prevented by Lord Vamana. But Lord Vamana also blessed him tht he will occupy that position in his next birth i.e. he will be the next Indra. Rgds, Mohan