I assume that something shiny happened by and I put the check in the envelope without signing it.
Back in the day when I still paid all my bills by check via snail mail (rather than online, the way God and Al Gore intended), I once wrote out an entire month's worth of bills (electric, phone, MasterCard, cable, gym, car insurance, etc., etc.) -- and then didn't tear them out of the checkbook, yet I dutifully filled out the remittance slip for each bill, put each one in its respective envelope, sealed it, stamped it, and probably even wrote my return address on the envelope, and then mailed them.
I didn't notice that the checks were still in my checkbook (SIGNED, even!) for a few days, at which point I had to scramble to find all the addresses of my various payees, write shamefaced apologetic explanatory letters, PUT THE CHECK IN WITH THE LETTER, and hope they'd be forgiving and not ding my payment as late.
IIRC, they were all understanding about it. And let me tell you, I was THRILLED when I was able to pay all my bills online.