1. Shitty bosses (this and the following can be found in any kind of company)
2. Being promised one thing and then being screwed over later. How many companies have screwed over their employees of stock options or those holding existing stock?
3. Terrible tech stack / less personal satisfaction -- especially if the startup hires lousy people and value code production over smarts.
4. Management defined engineering -- when managers make important technical decisions instead of engineers. ("We've signed up for service X, please integrate with them regardless of your thoughts on the matter").