This can happen if the lead submits the first progressive profiling form and on a second submission of the form (which would have different questions displayed), they change the email address (i.e. different person using the same PC or same person with two email addresses). Marketo will create a new lead record, but it will be missing the data that normally would have been captured in the first form. Look at the Fills Out Form activity and see if there was data submitted from a later progressive profile form, if so, this is what caused it. When this happens I create a smart list with an Anonymous IP filter and enter the anonymous IP address of the lead. Frequently, it will find multiple records and I can merge or manually update them as appropriate.