Could be solar flares. I also like to blame memory errors on cosmic rays. These can also randomly toggle CAPS LOCK. You can't ever be too careful.
What do you mean by "when testing"? Testing of T1 circuits can and often does involve setting various loopback options.
Almost sounds like there is some sort of automated T1/DS1 testing going on outside of the MX2820 M13 Mux? How are the MX2820 DS3/Network Interface and T1 Interface Looback options configured?