The dot com bubble was one of the major bubbles the world had faced. Although there is no exact reason of finding on what actually caused the bubble the mass investment was one of the basic reason behind the bubble. The imagination of investors that the technology market would never fail and thus making huge investments could be one reason.
Another reason could be due to the companies issuing public offerings without any stable strategy or plan. The companies were merely trying to portray higher earnings which often did not even exist. Perkins (1999) supports this in their paper 'The Internet Bubble' where they analyse how charisma of technology and immediate success created a revolution in the whole market.
A short documentary about Dot Com Bubble (.com/bubble)