He said that it was failure of the rubber o-rings.
He said that the probability of failure of those o-rings is small, however it happened to me :-(

I asked how on earth an o-ring could go wrong and then he reminded of the Challenger disaster in which the o-rings failed.

So however small, it happens...

He says that the company that makes the steering system is ZF (german company) and nowadays they make all the o-rings out of neoprene which has much lower failure rate.