I can't answer the part about what sets off a crash. AFAIK, it seems to vary slightly for each crash. They all share similar valuation problems though. If a company is worth $10 but it's trading for $50 then there's a risk that it will drift back to its intrinsic value in a bear market (aka mean reversion). When the most heavily traded companies are all overvalued then you see a situation where a large market can crash.
As investment firms realize that the investment isn't going through normal volatility, they'll sell off the shares and focus on other types of returns. The same goes for individuals but most will probably move after larger firms. A heavy sell off of a stock can cause the share price to drop. To some extent it probably happens in waves as negative events keep unfolding. That helps set the downward trajectory over the long-term.
Unfolding macroeconomic conditions like raising the federal interest rate will also have an effect like it does now in the current crash. That raises the interest rate for bank loans so consumer and commercial lending slows down. That typically ripples out with knock-on effects for the broader economy. There are other factors at play like inflation and supply chain logistics that have their own knock-on effects too.
There are trends in how to invest during a crash and bear market. Try looking for companies that are trading close to their intrinsic value that also offer a dividend. It's helpful to partition the companies by industry. Some industries are going to be fairly safe like healthcare, utilities, and defensive consumer products (e.g. toilet paper producers). Those companies are going to have steady business whether the market crashes or not.
On the other side of the coin, there are okay sectors and bad sectors. You can check the general valuations across the sector and what's going on across companies. For example, energy companies are hot right now because of oil demand. That's okay because it doesn't track the crash of the larger market. It's heavily dependent on oil value though, good luck predicting what happens to the price of oil over a year out.
A bad investment is something overvalued with bad future prospects. If you know people will be capital constrained and limiting big purchases, then cyclical industries like real estate might not be as good to invest in.
It might be helpful to think about companies in terms of size and market valuation too. A growth stock usually has higher price to book because it's invested in R&D for future growth. They may not be making that money right now but the price is higher because they should at some point if everything works. That hurts when mean regression happens and the market price snaps back to book value. Value companies make money now and have lower price to book. When they go through a bear market they'll be largely unaffected by mean regression and have an easier time getting consistent returns. They usually pay dividends which helps over time too.
I'm not sure if there's any consistent trend with how market cap size works in a crash or bear market. Check out some quarterly or annual economic outlook reports from major investment firms. They'll do the best at analyzing the current risk based on market cap if there is a trend.
Those bear market investment strategies should be put behind the caveat that long-term investing is a better way to go. The market ups and downs smooth out over time so trading based on fundamental analysis of a company usually works out much better than speculating on the current trends. With a long-term mindset you can be less stressed about news since you'll get less caught up with the short-term market conditions. Stay the course and all that.