For those looking for a motive to bombing Glorietta 2:

http://www.chairwrecker.com/column.php?col=790

From the article:

Just like the best laid plans of mice and men – and perhaps then favorite usual suspect Norberto Gonzales – the Glorietta 2 blast failed to provide the desired predicate for setting the stage for emergency rule.
The unexpected public reaction must have compelled the GMA regime to undertake a retrograde maneuver. They had to abandon the suspected original plan because it can only backfire on them at that point.
In such a situation, the first order of the day was to obliterate the public perception that the blast was a deliberate act of the government in order to attain a political objective. This was such an emotional issue that could incite and unite the entire country against GMA, making it her worst nightmare.

The methane gas accident story had to be told and sold. The methane gas accident story removes premeditated criminal intent from the equation. The only party that stood to be inconvenienced was the Glorietta 2, its owners and managers, as the blast could be the basis for charges of homicide and serious physical injuries resulting from reckless imprudence.