Why did the APR change?

All our staking contracts use a variable APR. This metric is determined by the number of tokens currently staked to the contract, the rewards are distributed across all stakers.

More tokens staked implies the same rewards are spread across a larger number, thus decreasing the APR. Same way when less tokens are staked, the APR tends to increase.
For most tokens, the APR is not fixed. The APR will change as conditions for validating transactions on the blockchain change.