Basically, we can divide reasons for losing into two categories: lack of knowledge and lack of discipline.
Some traders may claim that they were just unlucky and the market behaved in an unpredictable way, so this made them lose the biggest part of their budget. It is a total BS. Because of the fact that losses are integral part of trading.There is not a single trader who has never had losses. However, professional and skillful traders do use money and risk management in their trading routine. These techniques don't allow traders to lose lots of money irrespectively of the current financial situation on the market. Some people think that they don't need to learn anything and all they have to do is just click Buy/Sell button. Surely, such people will lose eventually.
Some people cannot cope with the psychological pressure or temtations. I guess that everybody knows that trading requires lots of self-control and psychological stability. However, not all of the people expect such things happen to them. So, these people may become blind from their emotions and ignore some evident things (like risk management) in the process of making trading decisions. Of course, they lose lots of money just because of psychological factors.