A question about emails: why does an email replied to become a "forward"?

My only guess is maybe the person hit forward accidentally, then put your email in?

Yep. @mowens is right. For whatever reason, the person forwarded back to you rather than replying.

I agree with the first two repliers- great answer!

A reply to or reply to all will prefix the subject line RE: If there was an attachment, it will NOT be included.
A forward will prefix it with FW: and it will retain the attachment.

Two options are that they hit forward so they could include the attachment and added someone to the email to send the attachment to and added everyone else in
or
They hit reply or reply to and changed the subject line to have FW: as opposed to RE:

@zensky I’m guessing you use Gmail, I have the same thing, everything is a forward, mainly because it includes the last message for some reason.

@XOIIO you can set the option in most emails on what the email will include (previous email thread) and what to make it look like (indent, etc..) when you reply or forward.