There are lot of reason's to occur the bugs in software application but most important factor is human factor.

1. Human being develops the software, so its natural that mistake would be happen in code/application. No one can perfect so any one can make mistakes while coding or testing

2.Sometimes client are non-technical & unclear related to requirement, developers develops the app accordingly after that he change the requirement ,Such frequent changes requested by the customer leads to errors in the application, Last minute design changes leads to many changes in application like redesign of whole application, rescheduling the task , scrapping of the work already completed etc.

3.A quickly written and poorly documented code is bound to have bugs. It becomes difficult to maintain and modify such code that is badly written or poorly documented.

4.Mis communication is also one reason of defects occurs in the software application.If we miss communicate to gather all requirement from client, so requirement should be clear & completed and it must to discuss each & every point regarding requirement with client.If requirement is not clear it will more chance for existing defect in software.

5.When project deadlines very come too close & time pressures come, mistakes are bound to come. Suppose if tester have to execute lot of test cases in a less time, then it will chance to get more mistakes in software application.

6.If tester don't have good testing skills & he not able to test with correct testing concepts so lack of testing skills which leads to defects in the system. Sometimes testers is not serious about his work & doing testing without giving any importance to it, so it gives results poor quality of product and still bugs remain in the software, So testers must have good testing skills & dedicated to his work.