Two things, both relating to the machine not feeding very well:
1. The front: the small portion of the cancel on the front with the killer bars at the end of the box shows that this piece of mail double-fed with another piece and got only a small portion of the cancel as intended.
2. The back: the machine also mis-cycled without a mail piece passing through it, probably fairly regularly, thus ink is applied to the rollers which roll along the back side of the mail pieces. The ink gets on the back-roller, then transfers (in reverse) onto the back side of the next few mail pieces which roll against it.
No doubt the odd length of the mailing contributed to the feeding quality.
I will disagree with Peter, it did not pick up the cancel from another piece of mail, or this would be obvious on a huge percentage of modern mail, which it is not.