The mains reasons for bug rejection are
Non-reproducible bugs – defect has not reproduced on developer pc, i.e a defect coming on your pc does not happen in developer pc’s
Test environment – mismatch of browsers, devices, or configuration of your application with developer.
Duplicate bug – Someone else has already reported the same bug
Misunderstanding the requirements – For any reason, if you did not understand the requirement properly, you would definitely look out for the misinterpreted requirement in actual implementation and when you would not find it, it would be a bug according to you, which will finally get rejected
Change in requirement – When testers are not communicated about requirement changes, more bugs will be reported and ultimately rejected.
Test data used – Test data used for testing was mismatching with a requirement
Improper bug description – If developer does not find proper description and required details in bug report, no matter how critical the bug is, it is expected to be marked as Rejected.