An alarming situation is one that's very worrying or frightening.
When someone says "it's an alarming trend," they usually mean that something is happening more and more often, but it's not a good thing. For example, if there are lots of robberies in your neighborhood, the police might say it's an alarming rate of crime. It doesn't necessarily mean something is physically loud or noisy - we just use this word to describe things that make us feel uneasy or concerned for our safety.
To call to arms for defense
Causing apprehension, fear or alarm; frightening
