The reasons behind the habit of blaming others

Loss of control: Some people panic when they lose control of a situation and so they try to restore the sense of being in control by blaming others!! So behind the act of blaming others fear might exist.

Controlling you: Some people try to control others by blaming them and making them feel that they are bad. If someone desperately wants you to do something he might blame you to change your behavior. Labeling you selfish, arrogant or snob by friends is one of the common methods for controlling others using blame.

They learned it from their parents: Some parents have failed to teach their children how to take responsibility for their actions and so they grew up as adults who blame others whenever something bad happens.

Refusing to admit that they are responsible: Some people can’t admit failures and mistakes so they blame others for them in order to escape from the responsibility. One common example is saying "I was late because the streets were crowded" instead of admitting that you should have taken the traffic jam into account.

Unable to accept what happened: People blame others when they fail to accept something that happened. Had those people learned how to properly accept events they would have never blamed others.

source: https://www.2knowmyself.com/Why_do_people_blame_others