This prophecy speaks of the first of the seven trumpet judgments and details a curious event with no seeming explanation of why it happens, or how.  One theory that has been put forward detailing what might cause this is perhaps a massive CME or Coronal Mass Ejection.  This would be different from the one theorized to possibly cause the great earthquake spoken of in Rev 6:12-17.  If this were to be triggered by a CME, it would need to be absolutely massive, so much so that it would press into the ionosphere (per the theory) and that, being about minus 150F (65.5C), would in turn be pressed into the stratosphere causing ice crystals to form which would in turn accumulate in large quantities and create hail.

The fire would be a result of the energy from the CME, as it's super hot ionized plasma energy which could easily set the smaller mass of the grass on fire all over the planet, and about 1/3rd of the trees, likely the most susceptible of them, and burn them to ash.  Also, anyone in the path of this who wasn't protected would die as well, either burned up like the grass and trees, or mortally injured by this event.  The source of the blood discussed in this prophecy however is uncertain.  It may come from birds who were flying at the time of this judgment and were killed, as the description says it was "mingled", meaning it won't be a full on rain, but rather that it will be sprinkled.

The other possibility is that the "blood" John was seeing was perhaps iron dust, captured by the falling ice, creating the appearance of blood covering the hail as it fell.  Another possibility, given the judgments that follow this, such as the burning mountain cast into the sea, it's possible that all of this disaster is the result of space debris, asteroids, comets, etc breaking up and impacting the surface of the Earth.  The busted comets filled with ice, or perhaps a mixed comet with both ice and rock, along with regular asteroids, would give the necessary results as indicated in the prophecy.

At this point however, given a general lack of information as to what it is and what causes it, these are only best guesses, but at the same time they are the only viable theories available at this time.  Either way the resulting judgment will be severe and painful and create much misery.  Especially the hail and fire.  Imagine how much damage that'll do, and the suffering and death that will result from its occurrence.  Also, another likely, although almost invariably overlooked consequence of this judgment will involve the food supply.  Regardless what season it is, there are always crops in the field.  Depending on what month of the year this hits will determine how much of that is destroyed.  Either way, this judgment is almost certain to further reduce, and quite dramatically I might add, the total available food supply of the planet, causing further starvation and death.