(יא) וַיְהִ֞י בְּנֻסָ֣ם ׀ מִפְּנֵ֣י יִשְׂרָאֵ֗ל הֵ֞ם בְּמוֹרַ֤ד בֵּית־חוֹרֹן֙ וַֽיהוָ֡ה הִשְׁלִ֣יךְ עֲלֵיהֶם֩ אֲבָנִ֨ים גְּדֹל֧וֹת מִן־הַשָּׁמַ֛יִם עַד־עֲזֵקָ֖ה וַיָּמֻ֑תוּ רַבִּ֗ים אֲשֶׁר־מֵ֙תוּ֙ בְּאַבְנֵ֣י הַבָּרָ֔ד מֵאֲשֶׁ֥ר הָרְג֛וּ בְּנֵ֥י יִשְׂרָאֵ֖ל בֶּחָֽרֶב׃ (ס)
(11) While they [the Amorites] were fleeing before Israel down the descent from Beth-horon, Hashem hurled huge stones on them from the sky, all the way to Azekah, and they perished; more perished from the hailstones than were killed by the Israelite weapons.
The appearance of barad, hailstones, seems very random, and is the same word used for the seventh plague while the Jews were slaves in Egypt.
We will look at what happened by the barad in Egypt.
(29) Moshe said to him, "As I go out of the city, I shall spread out my hands to Hashem; the thunder will cease and the hail will fall no more, so that you may know that the earth is the Hashem's."
והברד לא יהיה עוד. פי' לא יהיה מחדש ומה שכבר היה ויצא מן השמים אין עוד על דרך אומרו (פסוק לג) ומטר לא נתך ארצה ועמד באויר עד עת בוא דברו באחרית הימים והוריד אבנים שהיו על גב איש (ברכות נ''ד:):
והברד לא יהיה עוד, "and the hail will cease to be, etc." No new hail will fall, whereas the hail which had descended already will cease to be. Moshe's promise was essentially what is described in verse 33, i.e. that the hail and rain stopped in midair. According to an aggadah on Berachot 54, the function of that hail which was arrested in mid-air was completed in the days of Joshua when the latter was engaged in the conquest of the land of Canaan, when hailstones fell on Israel's adversaries.
This seems like a forced connection. Why do the hailstones from Moshe's time and Yehoshua's time have to be the same, though?
I will digress for a moment to get us closer to the answer.
(1) When King Adoni-Tzedek of Jerusalem learned that Joshua had captured Ai and proscribed it, treating Ai and its king as he had treated Jericho and its king, and that, moreover, the people of Gibeon had come to terms with Israel and remained among them, (2) he was very frightened. For Gibeon was a large city, like one of the royal cities—in fact, larger than Ai—and all its men were warriors.
Adoni-Tzedek was fearful because the Gibeonites, not because of Hashem, the One behind the strength of the Jews.
Let us also develop a better understanding of the barad.
(1) ויהי ברד ואש, there was hail and fire simultaneously, etc. The word מתלקחת is used to inform us that though water and fires are opposites, one of which is bound to prevail over the other in any encounter, in this instance they demonstrated the ability to co-exist. This was possible since both were performing G'd's will by so doing.
The hailstones that were suspended midair because of Moshe's prayer contained fire and water (who came together to serve Hashem), but never had their functions fulfilled due to their halt. And, what was the overall function of the barad?
למען תדע וגו'. פי' אין אני מתפלל עליך לצד...אלא להראותך כי לה' הארץ:
The only reason he [Moshe] prayed [to stop the hailstones] was so that Pharaoh would acquire greater respect for the power of Hashem.
The barad's function was to display Hashem's power. But, since the hailstones never fulfilled their function of displaying such power as they were stopped midair in the times of Moshe, they had to come back in the times of Yehoshua–where there were Amorites gathered together and were partially led by a man who feared the Gibeonites instead of fearing the Mighty Hashem–to do so. It would otherwise be unfair for Hashem to abandon the hailstones, which were the entities of fire and water who unnaturally came together to serve Hashem, as they never had the chance to fall and complete their purpose.
What a lesson this is for us, if we serve Hashem with everything that we can offer! This means that Hashem will not abandon us until we fulfill are tafkidim, our functions in this world, if we serve Him in the ultimate way of which the fire and water in the barad did!