What is it that will cause men's hearts to fail them for fear?

Luke 21:26 Men’s hearts failing them for fear, and for looking after those things which are coming on the earth: for the powers of heaven shall be shaken.

Isn't there only one other mention of a failing heart?

Psalms 40:12 For innumerable evils have compassed me about: mine iniquities have taken hold upon me, so that I am not able to look up; they are more than the hairs of mine head: therefore my heart faileth me.

Also, men have seen terrifying things, yet the reports have not necessarily been about their hearts failing them.

There may have been some, but isn't it more common that they saw terrifying things, yet survived?

There have been earthquakes, bombing raids, invasions, fires, and many other terrifying things.

But does Luke 21:26 possibly imply that men will see fallen celestial creatures coming on the earth?

Luke 21:26 the powers of heaven shall be shaken, but mark 13:25 words it slightly differently:

Mark 13:25 And the stars of heaven shall fall, and the powers that are in heaven shall be shaken.

Does this imply that the celestial principalities and powers will be shaken?

Will this happen at the same time as the following?

Revelation 12:9 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.

Will there be lots of fallen angels cast out with the devil?

Will they be terrifying to behold?

Will it be so terrifying that man's hearts will fail them?