Bugs Occur Because of the following reasons:-

1. Inexperienced developer:- Inexperienced programmers or programmers without proper domain knowledge introduce simple mistakes while coding. Lack of coding practices, unit testing, debugging are some of the common root causes that's why most issues get introduced at development stage.
2. Software complexity
3. When there is deadline/schedule pressure chances of Bugs increases
4. SRS (System Requirement Specification) not well defined
5. Frequent changes in requirement leads to develop product full of bugs
6. Designs not clear to the developers leads to develop buggy software
7. Changes in requirements not well communicated to the developers and testers
8. Poor Domain knowledge /history to the developers and testers
9. Poor testing
10. software development tool:- Visual tools, class libraries, compilers, scripting tools, etc. often introduce their own bugs resulting in added bugs.
11. Lack of skilled testers often results in buggy software