This can happen if the participant was logged in at the time the donation was made. When a user is logged in, the system assumes the donation is being made by the logged in user and so it will update the logged in user with the info entered by the donor. This is common when someone makes a donation on behalf of someone else using the same computer that they use to log in to their account. The donor would need to log out before making the donation to insure that a separate record is created for them and the participant is not changed. You can tell they were logged in by looking at the Interactions.
  1. Constituent360 > Constituents
  2. Search for the constituent
  3. Click View
  4. Select Interactions
  5. Look for the Donation interaction and look at the Solicitor
If the solicitor is listed as the constituent, then that means they were logged in. If they were not logged in, then the solicitor would say Unknown.