Ioan -

Two possibilities:

Outgassing from plating voids - this is generally a result of the board
fabricator having a problem during the activation and electroless deposition
processes.  The condition tends to get worse with efforts to touch up.
Occasionally this problem can be observed by external microscopic examination
of the plated thru holes.  Most often, it will require carefully prepared
microsections to find the problem.

Insufficient preheat - if you are using an aqueous flux, and fail to get rid
of the water and volatiles prior to entering the solder pool...... Guess what
- you've got bubbles or pockets of expanding gas in the solder.  This one is
fairly easy to spot - if you hear a pronounced sizzling sound as the board
moves over the wave, it's a pretty good indicator of this condition.

Good luck - Kelly