There are a couple of scenarios that can lead to a crash:
Hard Fork: if BTC splits into multiple currencies, there is the potential for one or both chains to lose a significant amount of value.
Security Flaw: if there is some security issue that comes to light about BTC that compromises the integrity of the ledger, that would most certainly cause a crash.
I think these two scenarios are the only reasonable ways to cause a crash.
If China or the USA made Bitcoin transactions illegal the price would probably plummet. How far is anyone's guess, but below $1000 would be a sure bet in my opinion. Over time it would rise again but if major exchanges were forced to close near-term trading activity would drop off sharply.
Granted this is a highly unlikely scenario and many people already successfully use Bitcoin to circumvent laws, but given the recent ransomware attacks and increased scrutiny of exchanges inevitably some out of touch politicians will propose a Bitcoin ban. That said, blockchain analysis is still far too expensive for the government to track down everyone's BTC (especially given the preponderance of quality tumbling services). So any ban would be useless towards stopping BTC activity should the core developers stick to their guns, so to speak.
Bitcoins price is tied highly to chinese regulations, as many people in china use bitcoin to move fiat out the country