I was doing an MSV LFR today, the part at the beginning with the big dog pull. I used arcane explosion, and got the message on my MSBT - 20 hits, 20 crits. What exactly is the chance of this happening?

My crit was around ~20% for simplicity's sake.

would it really be (1/5)^20 like I am thinking?

What i mean is that there were 20 dogs, and i crit all of them. I got 20 total crits.