How would an envelope from Citi (credit card provider) possibly become tampered?

Glue machine malfunction.
or
Stupid person trying to do theft of some sort.

Or the dampness caused the glue to no longer stick.
misread, thought you said it was damp :P

As @Zaku said, it could be a glue machine malfunction… or it could have gotten damp somewhere along the line between where it started and where it ended up and dried again. If your concerned that someone might have been trying some kind of identity theft kind of thing contact Citibank and alert them to your concerns. They don’t really like fraud and theft any more than you do.

@lilycoyote is correct in saying that if you think someone might have been trying to mess up with your envelope from Citibank, you should let Citibank know about it. This will help them to improve their mailing system against theft. At the same time, they will be informed that you received your envelope opened.