This soft shelled egg being laid late in the day is the next day's egg being laid early, and the reason it has a soft shell is that it hasn't spent enough time in the shell gland. You'll probably find that the hen who lays the softie won't therefore lay the next day.
My girls have done this before on occasion when thay have had a stressful day or a big fright ie a big loud thunderstorm, heavy equipment being used around their pen, the neighbours' kids going crazy on their trampoline.
Because the soft shelled eggs feel different to hard eggs, the hens will lay them anywhere but the nesting boxes. It's like they either KNOW that these eggs can break and don't want to foul up the nestboxes, or they DON'T know what on earth is happening and they don't realise they're laying an egg, so they don't go to the nestbox - either way, I've only found soft shelled eggs in the nestboxes a couple of times, and that has been when the hen has pushed them out during the night whilst sleeping in a nestbox (I assume the darkness would mean they wouldn't be able to see to get out of the box).
Anyway, I've kept the disturbance to the hens down to a minimum where possible (including shepherding them around the garden and distracting with treats when they lawn needs mowing or we have to do lots of bashing and crashing about in the garden) and the incidence of these evening softies has dramatically reduced. Just keep your girls calm and the noise around their pen to a minimum and they'll be fine.
It's nothing to worry about really in any case, it just makes them a bit uncomfortable while the egg is on it's way out (you'll notice they look ruffled and miserable just before one of these softies makes an appearance). They will also strain very hard whilst laying it.
Oh, and just keep an eye out for prolapse when a softie has been laid - they are MUCH harder to pass than fully shelled eggs.
Hope that sorts it out for you x
