The increasing number of active users might be one of the reasons. That thins out the voting power for the whales.
@arcange publishes daily stats about steemit. Among them is also how many fish/dolphins/whales there are and what voting power they have. When you compare the stats from 2 months ago to today, you will see that the voting power in the lower ranks grew by some 50% while the whales only added some ~2%.