My take. If in fact, very probable, that the main charge was a fertilizer bomb, i think the first blast contrail you see is the initiation charge. Small high explosive charge to detonate the larger low explosive charge. The larger fertilizer mixture is a slower burning charge, the detonation waves not as fast as the initiating charge, therefore the delay in initiating and slower moving from the point of initiation . High explosives, ie C3 C4 Det cord Simtex are used for shearing or cutting, whereas low explosives, ie TNT, Dynamite, Fertilizer etc... are used more for earth moving like blowing a stump or making a cratering charge.