Before HF19 the whales were running an experiment where they were not voting (or not voting as much). When they did vote, other whales were downvoting to counteract them.

The rewards pool was more full than normal, so each vote was worth more than normal.

After HF19, the whales began to vote again, which has resulted in the pool being drained somewhat - which makes everyone's vote worth less...

Now that we are in a phase that the pool is draining, our rewards for the posts that have not paid out yet are falling too - because the votes there are becoming worth less..

Eventually the pool will stabilize at the lower level, and you will see less fluctuation in your rewards..