You notice that new records in Marketo have the 'Website' and 'Company Name' fields populated with the domain of their email address.
Firstly this will only apply to net new records that fill-out forms. If the new record is imported or created manually then this would not apply.
This design decision has been a part of the code for many years where the domain of the email is used for the company name and website fields on the lead record. This feature cannot be disabled.