The entire of finance is linked. Those trading in the normal world of finance typically trade in more than one market. (bitcoin isn't normal, well not until recently, however the total daily trades are tiny)
If the entire angel market were to collapse as described it would cause a "contagion" that would whirl around the world.
Take for example uber with its stupidly large valuation. Real banks have invested real money into that company. Currently its horrendously over valued. Its a taxi company with an app. The only reason they are successful is that they have enough cash to under cut the world. They also have an obnoxiously selfish culture that generates PR via controversy.
If their credit ability were to be hampered, they would not be able to expand. This means they'd not be able to defy gravity and be forced to make a profit on the existing markets they have. cue massive implosion, all taxi services start to consolidate.
Once that happens it;ll start to affect other tech stocks and private investment.
once one or two unicorns fail because their underlying buisness model fails, it will lead to a total re-evaluation of credit for the entire tech industry.
This then filters into general stock. Add that to the wobbles from china, brazil, low price of oil and commodities and you have a nice crash.
The key thing here is that everything is interconnected. Like a open office if someone gets the plague, it spreads like wild fire.