One thing all stock market crashes have in common is debt. Stock declines force margin calls, which forces selling, leading to more declines, a vicious cycle.
Margin trading for individual investors is a recent development. Previously, individual investors were not allowed to open margin accounts. As this old article
explains, China brokerages became nervous of the margin debt at the peak. As soon as brokerages tightened margin requirements, the selloff began. This article from December 2014
goes into a little more detail on the recent history of margin in the China markets.
There is also Shadow
debt in the market, off-balance-sheet debt invested in the market, sometimes at a leverage of 3 to 1. Normal margin accounts are much more restrictive, about 9%. This shadow debt has been around for a few years now, but the latest boom is much more recent.
It should be noted that the China market has had huge booms and busts in the past, without the more recent leverage.