Premature birth Question

I had my son at 30 weeks and we spent 71 days in the NICU. The doctors couldn’t tell us why he came so early. I was healthy and so was he but he came fast and out of nowhere.

He just turned one and we are now thinking about starting to try for baby #2.

My question is did anyone have a very premature birth with no explanation? I’m worried that if we get pregnant again we will experience something similar.

It’s so frustrating to not understand why we had him so early and also makes me so nervous to try again.