A321 Sink on Approach

I just landed in Spokane but had a VS issue. The airplane started to sink on approach. It sank at a variant peak of 3100vs and came back up and did it again a 2nd and third time before I took control. Any reason that it would do this?

Aircraft: A321
Speed: IAS 180
Flaps: 2
Weight: 68%
Weather: 5knt
Trim: 30%

I had the auto altitude off and auto vs was set at -600.

This has never happened to me before.

I would assume its a one time thing unless you can reproduce it yourself. I have had my plane just fall out of the sky with no way to control it before, but only once.