Following are the main reasons of making the google adsense account disabled. Google will not allow any publishere to earn the money through illegal means and when they find that any publisher is doing this, they can disable their adsense account and all the accumulated money in that account is returned back to the advertisers.

Fraud Clicks is the Main Reason of Making Google Adsense Account Disabled

Google have many tools and methods by which they can detect the fraud clicks. Fraud clicks can be any type of clicks that are done just to increase the money of publisher and the clicker is not interested in the ADs in any way. Here is what google says about this:

"Click fraud is a subset of invalid clicks that are generated with malicious or fraudulent intent -- in other words, clicks that are intended to drive up advertiser cost or publisher revenue artificially. Sources for these clicks include, but are not limited to:

A publisher clicking on his own ads, or encouraging clicks on his ads

Users or family members clicking to support the site / publisher

Third-party programs with user incentives, such as paid-to-click services and click-exchanges

Automated clicking tools, robots, or other deceptive software

The same principles above apply to ad impressions and conversions as well. Some sources of invalid impressions include, but are not limited to:

Excessive page refreshes, generated either manually or automatically

Third-party programs with user incentives, such as paid-to-surf or auto-surf programs

Third-party programs for purchasing fixed amounts of traffic, e.g. "$10 for 1,000 page views"

Multilple Accounts - Another Reason of Making Google Adsense Account Disabled

Google does not allow to create more than one accounts for a single person. If you have more than one websites and want to monitor the progress of each website separately, then you can make use of "Channels". If google found that you have created more than one account, then both of your google acccounts may be disabled.

Illegal Content and Making the Google Adsense Account Disabled
If you have added the content on your websites which is not allowed by the google, then you may get the warning from the adsense and they may remove their ads from your website(s). If you will not take care of this issue and your pages still have the illegal content, google may disable your adsense account too.

Sites displaying Google ads may not include:

• Violent content, racial intolerance, or advocacy against any individual, group, or organization
• Pornography, adult, or mature content
• Hacking/cracking content
• Illicit drugs and drug paraphernalia
• Excessive profanity
• Gambling or casino-related content
• Content regarding programs which compensate users for clicking on ads or offers, performing searches, surfing websites, or reading emails
• Excessive, repetitive, or irrelevant keywords in the content or code of web pages
• Deceptive or manipulative content or construction to improve your site's search engine ranking, e.g., your site's PageRank
• Sales or promotion of weapons or ammunition (e.g., firearms, fighting knives, stun guns)
• Sales or promotion of beer or hard alcohol
• Sales or promotion of tobacco or tobacco-related products
• Sales or promotion of prescription drugs
• Sales or promotion of products that are replicas or imitations of designer goods
• Sales or distribution of term papers or student essays
• Any other content that is illegal, promotes illegal activity, or infringes on the legal rights of others