Air carrier technician reported hearing a pop sound and noticed another technician in the process of closing the thrust reverser cowl. The core cowl support was still attached to the thrust reverser and the support got damaged during closing.

Narrative

I was working an MEL for the Pack 1 fail and as I was finishing making sure all lines had been torqued properly I heard a pop sound behind me. I turned around and Person A was on the ground under the #1 Thrust Reverser (TR) looking as if to close it. I went over and supported the TR so he could see what was going on. At this time I noticed the Aft core cowl was open and still attached to the TR. I asked Person A to lock the TR up so we can look at the aft core cowl. The locking support for the Aft core cowl was cracked and twisted. I called the lead for the line Person B and notified him that Aircraft X was damage and we needed to start aircraft damage protocol. After getting off the phone we entered the aircraft and made sure all paperwork was filled out. I didn't hear from Person B so I called him and he was busy with other gate calls so I called the hanger supervisor and notified that Aircraft X was damaged and we needed to go into aircraft damage protocol. end of statement