As per my observation most common cause of startup failures are as follows .
No clear understanding of Start up / Entrepreneur venture with a differentiator / Me-too Entrepreneur venture . Positioning problem.
Not realising the importance of clear cut Business Plan , and thus not having one.
More enthusiasm in forming initial partnership /team rather than projects centric assessment of various characters and its role play.
Early in the venture Loosing technical sheen and totally getting engrossed solely in resource management and trouble shooting .
Not dropping the mask and seeking help when required from Coaches / Mentors.
While most are spot-on in initial assessment of needs of the market and gaps found , they are found lacking in the solution they propose . Moreover most become very possessive about the solution they propose . At times little correction in their solution also can make the project meaningfully viable , if not very scalable as per their original expectations .