As Mubasher Said, There might be some code running behind the scenes which is updating the data. Below information could help you to check.
1) If java script: Open the corresponding entity in Design mode, click on "Form Properties" on ribbon bar.
2) If plugin: Go to Settings-> Customizations->Default Solution -> Plugin Assemblies and see whether any plugins registered.
3) If workflow: Go to Settings->Processes
Thanks,
Mohan.B