1 People were overly optimistic about Tech companies
People were overly optimistic about the future of Tech companies that they sent their prices very high without much rational reasoning, See the Psychological factors that affect the stock market.
2 Many Tech companies started reporting losses
Many of the companies people had high hopes for started reporting losses. This led to massive selling. See why stock prices don't always represent the business value.
3 Prices of Tech companies were inflated
According to Investopedia In the year 1999, there were 457 IPOs, most of which were internet and technology related. Of those 457 IPOs, 117 doubled in price on the first day of trading.
4 Investors went after big ideas not business plans
Investors rushed into stocks that promised big changes to the industry without taking a look at their business plans. This made most of the tech stocks overvalued, See also Signs the market is about to crash.
5 Profitable companies were overvalued
During the Tech bubble there were some profitable companies that had real business plans but most of them were overvalued due to the craze that was taking place, See selecting winning stocks.
6 The use of metrics that aren't related to cash flow
Many investors started looking for certain metrics ,that had nothing to do with cash flow, to measure a business performance.
7 Bad accounting practices
Many companies started cooking the books and reporting incorrect numbers in in their financial statements. This made investors pour more money in those companies.
8 Many new investors jumped in
Because internet trading was relatively new. Many new and inexperienced investors jumped in the market. This led to much higher valuations for companies.
9 Many companies preferred growth over profits
Many Tech companies cared about growing instead of making profits. Those practices led those companies to bad financial situations.
10 Internet businesses were a relatively new idea
Because internet businesses were a relatively new idea investors couldn't really find a fair value for them. This led to speculation about the future of those businesses.