You are just starting out and the behaviour will change as some of the older items come back for Enlightened and Burn reviews and add to your overall workload. And if you forget them, then it is even worse.
I think maybe the good old “you are here” chart helps. Even if you are not exactly where the arrow points it makes clear why things are changing and will continue to do so for a while:
And if you are forgetting a lot of the old items, the chart would look even more dramatic.
Could it also be possible that you are doing all your lessons at once? If you start to space them out, either naturally or e.g. by using [Userscript] WaniKani Lesson Filter then you reduce the chances of being hit by one giant mountain of reviews all at once.