Cracked friction plates

I have been having a little trouble with the clutch on my 1977 KZ900, stock internals. After the bike gets warmed up, the clutch can get grabby and there is no smooth friction zone when releasing the clutch lever. It can make for some herky jerky starts at times, and I was getting pretty frustrated. After draining the oil this morning I took the clutch cover off, the spring plate and started to take the friction and steel plates out. The last two friction plates were cracked. Great. I have had new friction plates soaking in oil for a day now, so I started to reassemble everything and get the clutch buttoned back together. Refilled with oil and hopefully good as new.

Hoping this has solved my problem, but i can't ride it today to find out since it is raining.

Any idea what could have caused this issue, and would this be cause for my grabby clutch?

I would expect the clutch to feel grabby or otherwise abnormal with bits of clutch moving around in the basket. Ed

Are steel discs OK?

Steel discs were OK, I used scotch brite to scuff them up before placing them back.