Docking port assembly explodes for no reason

[Clear Air Turbulence]

Below is a picture of a docking port assembly that I am lifting up to High Orbit Hab, a simple refueling and space station about 1000 km above kerbin.

I am flying it up with a set of orange boosters and a small transfer stage consisting of spark, a non-stock liquid fuel and monoprop tank, some rcs thrusters, etc. Very simple

All goes well until I am in final approach, gently coasting in to space station for docking. Then the docking port assembly suddenly explodes due to overheating.

Another time it exploded while I was doing my ascent, just as I was in LKO

Why? We are not in the atmosphere, the only heat source is the sun... What could be causing this?

Edited October 29, 2015 by Clear Air Turbulence

[sal_vager]

This is obviously a support issue, moved!

[Stoney3K]

Try re-building your craft without that service tank, those service bays are known for causing all sorts of mysterious Krakens.