‘And when he had set the brands on fire, he let them go into the standing corn of the Philistines and burnt up both the shocks, and the standing corn, and also the olive orchards.'
Having prepared the jackals he then had them set loose strategically in different places for the greatest effect. The standing corn waiting to be harvested in the fields was burned, the shocks already gathered were destroyed by the fire, and the olive orchards too were set on fire causing great damage. Setting fire to standing corn was a regular way of retaliating against someone who had caused offence, compare 2 Samuel 14:30. The harvests of Timnah would be bare that year.
Similar things have occurred through history. Once thought of it was an obvious way of causing rapid conflagration and was eventually turned to service at sea with the invention of fireships. There was nothing profoundly religious about it. It was simply an easy way of causing great damage with the main culprits not being directly involved.
Samson's justification might well have been that the fields were common to the Philistine inhabitants of the town so that the produce was very much connected with the errant family.