Betraying God and turning away from Him ultimately destroys the very sources of a nation's joy. When people attribute the abundance and blessings they receive to idols and foreign powers, the result is the complete loss of those gifts. God specifically strikes the vineyards and fig trees. After the basic grain harvests, grapes and figs represent the finest of human food, serving as the ultimate symbols of joy during holidays, festivals, and Sabbaths [רד״ק, אברבנאל]. By targeting these specific trees, God turns the sources of their celebration into total desolation.
The reason for this severe ruin is deeply tied to the nation's profound ingratitude. The primary approach among commentators is that the people are compared to a brazen harlot who takes the dowry and wealth provided by her husband and treats them as payment received from her lovers [רש״י, רד״ק, אבן עזרא, מלבי״ם]. Another perspective simply views this wealth as a gift that was entirely misunderstood [אברבנאל]. These supposed lovers are the idols, the Baals, and the celestial bodies of the zodiac. In those times, the people mixed their beliefs, convinced that these foreign deities and heavenly forces controlled the yield of the earth. They worshipped them in exchange for agricultural abundance [מצודת דוד, שטיינזלץ, אברבנאל]. Because of this ultimate ingratitude, God takes away the fruit of the land, forcing the people to finally recognize Him as the true and only source of all the good they have received [רד״ק].
As a consequence, the beautifully cultivated orchards and fruit trees will be abandoned, reverting to a wild and untamed forest. The primary approach among commentators presents a dual understanding of what happens next. On a literal level, the destruction of the land means that wild animals will roam freely through the ruins, eating the abandoned fruit [רש״י, רד״ק, שטיינזלץ]. On an allegorical level, these wild beasts represent savage foreign nations and enemy armies, such as the king of Assyria or the Romans. With the exile of the Israelites, the land will lose its divine blessing, leaving foreign enemies to inherit the territory, plunder its wealth, and consume its produce [מצודת דוד, רד״ק, מלבי״ם, אברבנאל].