My guess is maybe just because they wanted to do some sort of trial.

Maybe because she is technically a Celestial Dragon?

She has always been treated as a slave child, not as a Celestial Dragon child. Imagine how they'd treat her after becoming a pirate.

Maybe Akainu got some sort of relationship with kuma? Still can't expect anything form Akainu

Remember that she escaped when Kuma still had free will so theoretically Akainu was probably scared that Kuma was going to rebel or go back on the deal with the elders. he was just afraid that they'd lost their leverage over Kuma.