The "Model categories" field on a CMDB product model record always get auto-populated with a model category when its associated asset is updated.
For example, a hardware asset record (alm_hardware) is linked to the model category "Hardware" and model "3com". When the hardware asset record is updated it also updates the "Model categories" field on model "3com" (cmdb_hardware_product_model) to put in the value "Hardware".
Caused by the OOB business rule named "Update model category".
This is by design and expected behavior to ensure that when an asset (i.e. hardware) is updated that its associated model and model category are all in synced.
If the behavior is not desired the OOB business rule "Update model category" can be deactivated:
Please note that there might be unexpected issues that can occur if the business rule is deactivated since the OOB design is now overridden.