Why did Adams sign the Alien and Sedition Acts?

As a staunch Federalist, President Adams was a firm believer in a strong federal government. To him, the United States was no longer a loose collection of states (as had been the case under the Articles of Confederation). Rather it was one country, bound by common laws and interests. America was becoming a nation, taking its place in the world alongside others, and, inevitably, it began to identify threats, both internal and external, to its newly-forged status. This is the background against which Adams signed the Alien and Sedition Acts into law.
The measures contained in the Acts were incredibly draconian. The Sedition Act, for example, cracked down sharply on any public opinion critical of the federal government. This led to the prosecution, and sometimes imprisonment, of prominent government opponents such as newspaper editors and politicians.
The Naturalization and Alien Acts dealt with the perceived threat of hostile foreigners. This legislation made it harder to become an American citizen and easier to deport those foreigners deemed to constitute a threat to the new nation.
Adams and other Federalists justified these measures on grounds of national security. At the time, American ships were under constant threat from French vessels. Also, the government was deeply disturbed by what it saw as growing outbreaks of domestic anarchy, namely, as the Whiskey Rebellion.
Not surprisingly, the opposition Democratic-Republicans were outraged. They regarded these punitive measures as a clear violation, not just of the Constitution, but of the spirit of 1776 and the fundamental concept of liberty on which the Declaration of Independence had been founded. As the Supreme Court had not yet arrogated to itself the right of judicial review, there was little it could do to hold back the growing authoritarian tide.
Although most of the legislation was repealed within a relatively short space of time, the spirit it embodied remained. The authoritarianism of the Adams administration has lived on in American history ever since, manifesting itself in egregious violations of civil liberties such as the Red Scare of 1919, McCarthyism, and the internment of Japanese Americans during World War II. Indeed, it is instructive that the FDR Administration used the provisions of the still unrepealed Alien Enemies Act of 1798 to carry out this fundamentally unjust policy.
https://www.ushistory.org/us/19e.asp