And how does corruption happen?

You just need the right conditions.

From the above link:

• Concentration of power in decision makers who are not directly accountable to the people as often seen in non-democratic regimes.
• Lack of government transparency in decision making.
• Costly political campaigns, with expenses exceeding normal sources of political funding.
• Large amounts of public capital involved in a project.
• Self-interested closed cliques and "old-boy" networks.
• Weak rule of law.
• Weak legal profession.
• Minimal freedom of speech or freedom of the press.
• Poorly-paid government officials.
• Apathetic, uninterested, or gullible populace that fails to give adequate attention to political processes.
• Absence of adequate controls to prevent bribery or so-called "campaign donations."

Uh-oh.

We may be in trouble.