Hm, I see what you mean...
Perhaps after your hitpoints are depleted you start getting fires, EMP shorts, engine flameouts etc. but as you've said, no immediate boom.
At this point we say that the reactor is exposed or containment of the anti-matter field is unstable. Or maybe even the internal 'energy shields' are failing.
Now whether or not you get a final 'explode' depends on:
Alpha damage of each shot hitting the vessel
AND/OR
Cumulative damage recieved inside of a frame/second.
That is, your hitpoints are at zero, a stray MG isn't going to kill you...but lots of MGs doing say 500dps combined? yes that causes an explosion.
Or say you get hit by one big hellbore shot, definitely a big boom, a single small pulse laser shot won't.
Then a big boom