I have a decent working knowledge of physiology and bio mechanics

I have a decent working knowledge of physiology and bio mechanics yet still don't always understand how the body works. I am in a particularly fine phase in my running life at the moment, I'm fit and fluid. Tuesday evening is the weekly club run and I set off feeling good, I run strong at the front of the pack and I'm comfortable there. I even feel that I'm pulling the group along at one point. However after five miles I start to tighten up, I can feel it in my lower back and hips. After seven miles I leave the group run and cut for home. I sleep light and restless that night waking several times, my legs sore and stiff.
Why is this? I have run further recently and in hotter times of the day and had no adverse reaction. From time to time I seem to get this and it does seem to relate to my lower back.
On the plus side, since I switched from conventional shoes to minimalist shoes my recovery is swift, by lunchtime on Wednesday my legs are fine. Three years ago I would have been sidelined for days.