Trades get declined on their own

For a long time I wasn't sure if this was a bug or if I accidentally declined trades sent to me but it just happened again and this time I paid attention to what I did: I read a trade that was sent to me (i.e. the bold fonts were replaced with regular), and then checked all the mails and pressed delete to get rid of battle reports, then closed the mail window without unchecking anything. By the time I opened it next, the trade I read was declined (another one I didn't read was still there).
Not sure if it happened because the making it read thing or because it was checked or the deleting or all of them or all of those together.

Anyone had the same thing happen?

how long were these trades there for, as they auto decline after a set amount of time (somethink like 8 hours)