Explosive Packages Found at Heathrow, Waterloo and London City Airport

"Explode" means "burst or shatter violently and noisily as a result of rapid combustion, excessive internal pressure, or other process", whereas according to the article "these packages only appear to be designed to start a very small fire - the one that went off just melted part of its own plastic envelope..."

The correct term is "incendiary". But this seems so small as to practically put it in "prank" territory - since they only go off when opened, there's not much risk of it starting a uncontrolled fire unless someone happens to open it next to an open container of gasoline.