It looks like clicks on scrollbars of <select> elements (such as the Component field in Bugzilla new bug form) are handled by the scrollbar then passed to the webview. Thus rapidly clicking on the scrollbar will cause the text of the page to be selected. (svn revision: 48333)
I can reproduce with epiphany 2.28.0 (using webkit 1.1.15). I was clicking the 'down' arrow on the scrollbar in a text area, and suddenly the whole thing got selected.
Clicks are no longer handled this way, since interior scrollbars are "fully-fake." I could no longer reproduce this issue, so I will close it.