They, is a reference to all who have rejected God, Christ, doctrine and the spiritual life as God has designed and defined it. And more specifically, they, is a reference to that final generation, that will live in the last days of human rule on this earth, the Tribulation.
We know that the tribulation is yet future, and will begin when our Rapture occurs, and will last for seven years. The end of which will be the most horrifying period in all of human history.
Judea and Israel in general is a mountainous country. And within the mountains and hills, there are many such caves and crevices and places into which a person can go for safety and hiding. And this provides us with a descriptive example of just how terrified people will be, and to what lengths they will go to hide from Gods wrath.
At En-gedi, a cave is mentioned where David with six hundred men hid himself from Saul in the ‘sides’ of the cave system, 1 Sam 24. Caves or dens were sometimes constructed for refuge or defense, Judg 6:2; 1 Sam 13:6.
So in times of extreme trouble, such as in an invasion, people would run and hide anywhere they could find what they thought would provide them with some semblance of safety or escape. Refugees are always on the run just trying to stay ahead of the advance of a murderous invading force. But people hide in basements, and closets and in attics and street drain systems and such.
When He arises, is an expression that means when the action begins and continues. In this case it will be Gods judgment, a series of actions brought against unbelieving and wicked humanity, which actions will last for many weeks and months and years during the ultimate period of judgment, the Tribulation. The worst of which will occur during the last half of the Tribulation, which will last three-and-a-half years.
Having difficulties such as storms or invasions or crime or atrocities is bad enough when they last for just a few days or even weeks, but when they continue on and on and there is never an end, then that will be the worst possible experience to have, ever.
For to have the earth literally shaking beneath and all around you, with no stopping, makes it very clear that something if really wrong.
That earthquake in Japan pasted for six minutes, which is a pretty long time for an earthquake, but then a half an hour later the ocean waves came in and buried several coastal cities. And, they just kept coming and coming. But then that disaster finally came to an end, just as the disasters in Indonesia and Haiti and other places ended.
But, during the Tribulation, as we have already studied in our Revelation study, the disasters and horrible circumstances will commence after the Rapture, and they will continue and escalate for the entire seven year period, being extreme and beyond extreme in the latter half of the Tribulation. During that seven year period, very nearly the entire population of the world, will die. Not sure how much more extreme and terrifying things could possibly get on this planet.