Every year too many people

English as a Second Language English Vocabulary, Grammar and Idioms
1

Every year too many people are killed, usually triggering the avalanche that buries them.
What is the meaning of the bold part?

2

Usually causing/setting in motion.

3

Every year too many people are killed, usually triggering the avalanche that buries them.
I think underlined part is a modifier. am I correct? If then which part is being modified by the underlined part?

4

Sorry, this question is beyond my ken I’m afraid.
Hopefully somebody will get back to you on this one.