Can you explain the phrase 무슨 일이야
무슨 일이야
How does the phase mean whats up?

What does the phrase mean 일이야? Is root verb 일이다?

Question

Can you explain the phrase 무슨 일이야
무슨 일이야
 How does the phase mean whats up?

What does the phrase mean 일이야? Is root verb 일이다?

Answer

It means 'what's this/it about?'  
무슨 means what/which/what kind of
일 means work/thing that's going on/thing that's to be done
이야 (i-ya) is the casual form of (i-e-yo), meaning 'is' - the more polite way (i-e-yo) can be used to an official person who stops you to ask you questions, like 'What's this all about/in relation to?'
:)

Answer

일이야 = 일 + (이)야 
일 [noun]
The root "verb" is "이다", which is part of 이야.