There might be a handful of reasons for your competitor to rank above you in Google search results. But it’s important to know that just because they are ranking above you does not mean that you can not rank above them a few weeks or months after.
Some of the possible reasons for your or your competitor’s ranking might be some or one of these: keywords placements, backlinks, technical SEO errors, page speed, domain authority, content quality, Google search rules/updates, security, mobile-friendliness, user experience, domain age and more