Check below the link for a thought-provoking paper published this year in the journal Assessment & Evaluation in Higher Education titled "Can professors buy better evaluation with lenient grading? The effect of grade inflation on student evaluation of teaching":
"At many universities, student evaluations of teaching (SET) are used for determining promotion, tenure, and other financial benefits for professors, which gives incentive for them to try to increase their scores. However, previous research, mainly based on US data, indicates that SET scores not only depend on teaching effectiveness and quality, but also on several other factors, most notably on grades...Our findings suggest that increasing the grade of a student by one will cause them to give approximately 0.2–0.4 higher evaluations..."
Of course, we cannot forget that in the UK the professors who give lower grades risk being investigated https://www.timeshighereducation.com/news/lecturers-face-investigation-if-average-mark-below-21 which means a more lenient grading approach becomes a favorable choice for many, creating a win-win scenario. The students get happy and in return, the professor gets better evaluations from students, and in this way the professor also avoids investigations! There´s only a "minor problem", the integrity of the evaluation process.
Update on December 2021 - In the conclusions of a recent paper published in the very same journal one can read the following: "use of online student evaluation of teaching as a platform to harass, offend and, at times, menace teachers in higher education" https://www.tandfonline.com/doi/full/10.1080/02602938.2021.2012643
