SR22 incident at the range today

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May 2, 2018
We went to our local range today to shoot our .22's
I shot 3 shots with my SR22 and on the 4 th shot the slide and spring flew off.
The RO shut down the entire range and retrieved them for me and laid then on deck.
I checked the gun and saw that the take down lever was in the open position (perpendicular to the frame). My first thought was OMG I forgot to close it after I last cleaned it. I then tried putting my finger inside the trigger guard and I could not do that without touching the take down lever.
I have never had that happen before and am at a loss for what happened.
Any thoughts ?