The first thing that comes to mind is post or topic removal (not locked but deleted topics). That is how it happens for most forum formatting. On some boards, they let users keep post counts even if certain topics have disappeared.
Anyone who breaks the community guidelines that gave hearts to people in the past could have those removed. Example:
Userx gives MemberY 3 hearts.
Userx does something to break ToS or the guidelines; gets banned. OR Userx deletes their account.
All hearts given by Userx are removed, including MemberY's 3.
And never rule out the glitch factor. Hopefully the reason behind it gets figured out. If not, those three will be replaced soon enough. (: