Vegging plants autoflowering

I've got a nice outdoor grow going here. Something strange is happening. My vegging plants are starting to auto flower. These are not strains that normally do this. I have been growing these strains for a while so I know them well. It is the first time I've done a grow from start to finish completely outdoors. The grow location has great sunlight for the entire day. First light is around 5AM and twilight is still around 9:00 PM. Some started flowering around the end of June (pretty close to the vernal equinox) so I know it's not a photocycle issue.

The temps can get fairly warm in this area..up into the 90's, but there is good air circulation, the plants are growing rapidly and there are no signs of heat stress.

I'm just going to let them do what they need to do, but it's pretty baffling. Anyone have any ideas why this could be happening?

My outdoor plants showed preflowers in very early June. They were not actually flowering fully, but showing that they were mature. If you haven't grown this strain outdoors (or over such a long period of time to larger sizes) it may be why you haven't seen them do it before.