The status "Editor declined invitation" means that the Associate Editor who was in charge of your paper has declined the invitation. Since the previous status was “Reviewers assigned,” it is clear that the AE had initially taken charge of your paper and had sent out reviewer invitations. However, probably he/she chose to decline invitation later due to some unforeseen turn of events. It is possible that some urgent work cropped up due to which he/she felt that he/she would not have time for your manuscript.
Another possibility is that the editor later realized that he/she might have some conflict of interest with your paper. Whatever the reason for declining the invitation, the reviewer invitations that the editor has sent out will probably remain valid and the new editor who will be subsequently assigned for your paper will take them forward and follow up with the reviewers. This is usually the case when an editor changes midway through the journal evaluation process; however, one cannot really be certain, and a lot will also depend on the new editor who is assigned. Things will be hopefully be clearer after a few more status changes.