Board Thread:Lion Guard Discussion/@comment-43313582-20190801012822/@comment-35360394-20190801014214

They know Scar's legend and first Janja didn't trust Ushari about sommoning Scar's spirit. Ushari sort of eavesdropped the facts of bringing back Scar while Rafiki spoke with Kion in the lair. But then what might have been the reason for the Janja's clan to surprisingly changed sides is them suddenly realizing Scar isn't actually willing to share stuff, and then Janja's clan finally understood the danger of listening to Scar's plans.

Past of Janja's clan and the future this would be my guess of why they're now reformed.