Drop D tuning all by itself

[quakana]

So I leave my guitars in my finished basement in the summer I control the humidity and in the winter my woodstove (pellets) is on to heat the place.
The guitars hand on the wall and sit in stands down there. After a couple of weeks on not playing them the tuning is all off and most of the low e strings are at D. I know about expansion and contraction of woods and warping. But the necks do not look any different concave or convex wise. They are just outta tune. Action is the same. intonation peferct. Boards are always treated with lemon oil

I am wondering if it is the strings due to slight temperature change when it gets real cold outside. The room is always at 65-70 though

Any ideas all you freakin genius out there