There is no such thing as a stablecoin or stable coin however you want to say it. There are 2 big reasons for this. The first is they are traded. Nothing that is traded in the crypto world (and probably any other) is stable. At any time a big actor or event can drive the price up or down. We have seen this with the Steem Backed Dollar. It's supposed to be around $1 but due to the run up in crypto prices it hit over $12. This was due to people just buying anything without knowing what it was. So in this case an event. The same thing could happen with one big actor buying or selling a bunch at one time. This could cause more buyers to do the same thinking something was going on or a panic sell. Same goes for any other stablecoin like tether. We say a bit of this last night when it lost it's peg.