Each survey serves a specific purpose for each research company.
If the answers that you provide are not consistent with the type of person they are looking for, the data will be excluded from the final results. As a result, the survey will end and partial credits will be awarded.
The survey suddenly ending is not always based on your demographics, and can be based on a set of responses that you provided during the survey.
For example: Some surveys may ask the same questions twice, to make sure that you are paying attention. Conflicting answers will result in disqualification.