Three things actually occur here.
1. Companies pay the employee who left the review if they can identify them. $1,000 to the former employee to take the review down, along with an NDA.
2. Companies push Glassdoor to validate the reviewer was actually employed by the company. Some reviewers may not respond to Glassdoor’s confirmation requests, resulting in the review being taken down.
3. Companies’ HR or Talent Acquisition employees write reviews themselves and promote internally to other happy employees that adding “honest reviews” will help accelerate the company. Basically indirectly fishing for 5 star reviews.
Any startup with more than ~300 total employees (current + former) generally has attention to their Glassdoor ratings. A local company I know of is a huge offender to removing negative reviews and flooding their page with 4.5/5 star reviews in quick succession.