During a period of deep political and social crisis in the Kingdom of Judah, a large faction of conspirators grew exhausted with their local government. Living under the reign of King Ahaz, these citizens developed a deep resentment toward the royal house of David and sought to replace it with foreign powers that appeared far more impressive and formidable. The prophet rebukes the nation for this betrayal. The primary motivation behind their rejection was Judah's military weakness, which stood in stark contrast to the strength of the neighboring Kingdom of Ephraim [מצודת דוד, רד״ק]. The conspirators despised the Davidic dynasty and hoped to install a new king, a plot that struck terror into the hearts of the current king and his ministers as news of approaching enemy armies spread [שד״ל].
The local leadership is compared to the waters of the Shiloah. While some view this as a symbol of Jerusalem itself, the primary approach among commentators is that it represents the royal house of David [אבן עזרא]. The Shiloah is a small spring, often identified as the Gihon [רש״י, מלבי״ם, חומת אנך]. This imagery is highly deliberate, as the kings of the Davidic dynasty were traditionally anointed beside this very spring [מלבי״ם]. These waters are characterized by their slow, quiet, and gentle flow [מצודת ציון]. This calm movement perfectly captures the moderate, humble, and peaceful style of leadership embodied by the house of David [מצודת דוד, ביאור שטיינזלץ, מלבי״ם].
However, it was precisely this gentle nature that earned the people's contempt. They viewed a quiet, restrained government as proof of military incompetence and an inability to strike back at their enemies [רד״ק, שד״ל]. Furthermore, the citizens mocked the modest lifestyle of their leaders. They looked down upon figures like King Hezekiah, who contented himself with simple meals and devoted his time to studying the Torah, preferring instead the flashy, extravagant feasts of corrupt rulers [רש״י]. Driven by a desire for dominance and raw power, the people rejoiced in the leaders of rival nations. They actively desired the aggressive, showy rule of Rezin, the king of Aram, and Pekah the son of Remaliah, the king of Israel, wishing that such men would rule over Jerusalem [מצודת דוד, רד״ק, ביאור שטיינזלץ].
Because the nation rejected the gentle, life-giving flow of their own leadership and lusted after the aggressive power and material wealth of foreign kings, God responds with a direct, measure for measure consequence. Instead of the quiet, manageable waters they despised, He will unleash upon them the violent, overwhelming floodwaters of those very enemy armies to wash them away [רש״י].