Why is my credit score going down?

Bad financial management leads to a bad credit score. Your credit score can vary depending on the credit reporting agency, and lenders that request your credit scores and credit reports. Your Individual criteria can determine your creditworthiness, as well as your chances of approval for various loans and credit cards.

FICO® and VantageScore® both consider similar criteria to calculate your credit score but give each factor slightly different importance. If the factor holding the most weight is not managed well, the chances of your credit score dropping are relatively higher than criteria that hold lesser importance.

Some of the factors that can bring your FICO score down significantly are:
Payment history: 35%
Amounts owed: 30%
Length of credit history: 15%

This is how VantageScore weighs its factors:
Total credit usage, balance, and available credit: Extremely influential
Credit mix and experience: Highly influential
Payment history: Moderately influential

In a nutshell, missing payments over a period of time, high credit utilization, and currently-owed debts contribute to a lower score.