The reasons behind filing an opposition vary. No matter what the case, the party filing has to be one which believes they will be damaged if the mark is registered. One possibility is likelihood of confusion. Even if the examiner did not raise the issue, another party may during the opposition period. In addition, one could argue they had made prior use of the trademark. Examples of prior use include advertisements or using the trademark publicly. Descriptiveness and genericness may also be argued if the opposing party thinks they will use the trademark selling the goods which are described by the mark. Similarly, misdescriptiveness and deceptiveness can be argued by parties which are competing with the applicant mark. Things that cannot be used to file an opposition include disparagement and unfair competition. Opposition can be a valuable way to protect your trademark or it can pose a major hurdle to registration. In either case be knowledgeable about what is being asserted and consider hiring an attorney.
This blog is not legal advice and is not specific to your application. You should always consult an attorney.