Ok - I got to the bottom of this..
The problem was that some of the func_explosives had a DMG value too large (for fritzbot.)
I didn't have the inclination to try and find what the limit is, but 400 works, and 800 doesn't. Draw your own conclusions.
I presume the same may be true for fritzbot ET.
400 or 800 are quite large, but I wanted an explosion of an oil refinery to blow you off your feet unless you were some distance away.
So now i guess you can get closer before you lose your hat.
For reference, mp_tram uses DMG values of 75, so 400 is probably large enough.
If anyone has any simillar problems with their func_explosives getting triggered at startup, then check the DMG value.