What did Plato think could cause democracy to turn into tyranny?

In his work the Republic, Plato argues that democracy is inherently unstable and prone to devolving into tyranny due to the unchecked influence of demagogues and the corruption of the populace.1

Plato observed that in a democracy, the poor gain the upper hand and leaders—whom he termed demagogues—distribute a "peculiar kind of equality" to everyone impartially. In this environment, political leaders prioritize flattery over wisdom, fawning over the youth to avoid appearing despotic. These leaders systematically plunder the propertied classes to distribute spoils to the people, fostering an atmosphere of confusion and corruption.1

As this state of disorder deepens, the society becomes ripe for a tyrant to emerge. Plato described this transition as a process where the leader eventually transforms into a "wolf" rather than a man. To maintain power and distract the public from their growing discontent, the tyrant eliminates potential rivals and initiates wars, eventually revealing to the public the "monster they have begotten".1