Shell Less Egg Incident

Hi,
I'm new to this forum and new to chicken keeping. I have three hybrids who have been laying for approx 4/5 weeks now. All was going very well. I had three eggs this morning as usual. This afternoon however, I let them out to roam around the garden and in the process of trying to get them back into their run, I noticed one eating an egg in the garden. I then had a look around for a shell and found a further two shell less eggs elsewhere. I am totally perplexed as to why this would happen and why they would all three of them lay an underdeveloped egg at more or less the same time (they were only out for about an hour).
There have been three changes to their "routine" in the past 24 hours - I changed their food to layers mash mixed with growers mash (same brand though). I added some cider vinegar to their water as recommended by my poultry store and we have covered the run to try to avoid the mud bath that's developing there so the run is darker although sides are still uncovered. Can anyone suggest what might have happened? I guess I have to wait and see if they lay as normal tomorrow although I am expecting it to be later than usual if they do.
Many thanks

Hi, Yes they do have access to grit but not oyster shell (do I need to get them some?) It was more the fact that all three of them did it at the same time and in the garden when they always lay their eggs inside the coop/nesting box. I know that shell less eggs can happen. It was just so out of character. I wasn't sure whether to be worried or not. Will see how they go for the next week or so as you suggest. Thanks

I was conscious of a lot of dogs barking a few doors down from us yesterday when they were outside. Perhaps it was the noise that disturbed them.  They seem quite nervous generally and the slightest gust of wind sends them flapping across the garden in fright. Will get some shell for them and see if that helps. Thanks

phew thats good news 

I seem to have had the same problem. Had my girls a couple of weeks now and all been well.Yesterday one of those police helicopters hovered over head for ages and scared them witless so much so they nearly jumped into my arms.Could this have caused the soft shell eggs I had from two of them this morning?