Outpost75
Not being an artillery guy I've no 1st hand experience with S.E.E. in artillery rounds. However, I have read and studied the matter in conjunction with the S.E.E. phenomenon in firearm cartridges. Much of the criteria for such to occur in either is the same with the exception of the initial ignition stage. In the artillery rounds, as you mention, the initial cause seems to be the "black powder igniter contained within the booster or flash tube being fractured, causing delayed ignition". In the firearm cartridge the delayed ignition is caused by primer "flash" not igniting the powder (usually a slow burning powder with heavy deterrent coating to control the burn rate) sufficiently that the powder begins burning progressively. The primer "flash" ignites the powder just sufficiently that is "smolders" so to speak while the force of the primer flash pushes the bullet into the chamber throat where it momentarily stops or sticks. The powder then starts to burn and the pressure rises to a catastrophic level before the bullet can move quick enough the decrease pressure via the expansion ratio. Note in most S.E.E. events the bullets do exit the muzzle. They just don't do it quick enough and something else gives.
As to "high pressure excursion" perhaps the technicians at Hodgdon changed the definition to fit, I don't know. They should however, as tomme boy mentions, be very familiar with S.E.E. because the earliest reports are most notable from the early '50s with the use of H4831 in 25-06 and other "over-bore" cartridges. I have found several probably S.E.E events occurring prior to that in some writings including Hatcher's or Whelen's. The phenomenon just wasn't recognized as such back then.