A few points; sorry for the list format (done whilst having my dinner!)
1. Do windscreens crack for no reason? No.
2. There is always a cause. One of the biggest misconceptions in the trade about 'spontaneous' breakage is stress fractures. There's nothing spontaneous about it and there is no sorcery at work; closer inspection will reveal a (or the) cause. Stress cracks are extremely rare.
3. Was the replacement windscreen genuine, AR branded glass? If it isn't, the likelihood of it not being as good as an OE version are very high.
4. Claiming for a windscreen is a claim. There is no other name for indemnifying loss. The inference is towards whether or not it will impact on your NCD; some insurers will, most do not. However, it should be confused with loading your premium on renewal.