They have to assume it could happen again because it's safer to prepare than not. You are at a slightly increased risk than ladies who've never had one BUT that doesn't mean it will necessary happen again. It really depends on why it happened the first time, eg infection, a simple uti can cause your waters to break early. Was it a preterm prom?
I had a prom with my first at 34+4 in 2012 no reason found, he was born 8hrs later. I'm now 35+6 with my second and anxiously holding on :)
I had it at 37 weeks with no cause. She ended up being born 23 hours later - I didn't go into labor. They started induction at 13 hours later. She was in distress so had to deliver her at 8 cm and almost rushed to c section twice as I wasn't progressing. She was born with pneumonia. And they didn't catch it till an hour later even with voicing my concerns at delivery/birth