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This only happens if someone has previously identified her/himself as another lead. This doesn't only mean filling out a form. It may mean clicking a link in an email originally sent to someone else.
One of the most common ways it happens is with the link clicks like Sanford Whiteman talks about. Hyperlinks in emails are decorated with tracking data appended to the end of the URL. That's what's used to identify the click activity in the lead record, and each tracking token is specific to the lead record the email is sent to. Now, let's say the lead it's sent to forwards the email to other people, or even just takes the hyperlink out and sends it separately in some other way. The tracking token appended to the URL goes with it, so anyone clicking that link will be identified as the lead specified by the tracking token.
It's a fairly common issue, so our Product Docs team created this doc showing how to disable that tracking: Disable Tracking for an Email Link - Marketo Docs - Product Docs
Just note that if you disable tracking, it disables it for all link clicks including the first initial recipient.