When you did this, I suppose you cleaned throrougly the combustion chamber and exchanged the old plug for a new one in order to prevent that aluminium particles contaminate the plug... (seen many people doing this and not cleaning the combustion chamber nor exchanging the glow plug).

What I suppose has happened is this. You have suffered a conjuction of events who when combined ends with a grenaded engine.

- Probably your conrod was worn and started to develop some play. This helps too to blow glow plugs.
- Doing that scoop on the combustion chamber reduced your compression ratio, so, maybe you removed 1 or 2 shims in order to regain the lost compression. This is very dangerous to do is 'as is' because in fact you're placing near two solid things and one moves at a very quick pace (the piston). If you not take some care with this, you can end with a crushed piston when everything is warm.

I did this, but removed some material too from the combustion chamber (described it on a previous post) in order to avoid a too near piston of the CC and to maintain some squish band to (the 'flat portions' on the CC aren't really flat), are made in a small angle (around 2 or 3 degrees) making some kind of small ramp.

- Tried and needed to switch to a colder plug too (sometimes going a number higher), other times needed to switch from a Hot to Cold body maintaining the same number... You're putting the plug near and receives much more heat, so you need to take this into account.

Seems easy to do, but in fact, there's much to gain doing this, but some things need to be done before to avoid detonation and grenading the internals.

BTW: An engine who blows two plugs in a row is a serious candidate for a teardown, cleaning and inspection, you never know where went metal particles of the plug wire....