So certain events may arouse (for example excite or worry) your horse somewhat, but he doesn't react in an extreme way. If several things happen close together, it all gets to much - and he 'explodes'.
This may sound really obvious, and most of us are aware of examples of this - say you want to compete your young horse - you don't introduce him to travelling in a trailer, going to a strange new place, and competing all in the same morning! At least you probably don't do it twice ...
The smaller incidents can confuse people. To take a fairly recent example, we have a young cob called Freddy. Access to Freddy's yard is blocked one day, and the farrier is coming. His owner takes him down to a quiet farm track for the farrier to shoe him. He's a little fidgety, but 'fine'. The farrier arrives, and Freddy is a bit more 'difficult' than usual, but can still be handled. As the farrier is putting on his second shoe, a tractor comes into view. Freddy 'explodes' - neither his owner nor the farrier can hold him, and he takes off for home, half-shod. The owner is mystified - he's never behaved like that before.
On initial questioning, Freddy is reported to be 'fine' with the farrier usually, fine on his own, and good with traffic. With more detailed questioning, we find that Freddy is actually:
* a little worried when separated from other horses, a bit nappy hacking out alone but manageable
* a little uncomfortable with the farrier, but the farrier always manages to get him shod
* a little worried by tractors; he will 'scoot' past them on the road, then settle down.
So - it's trigger stacking.