Its not a significant wider issue for Samsung phones but screens do sometimes mysteriously crack. You don't need to drop it though, just a little flex if you say sit down with it in your pocket can put the corners under stress and a crack to develop.
In theory you should be able to get the screen replaced by Optus or Samsung. Your consumer rights do protect you from product defects causing issues. In practice I suspect Optus and Samsung will heavily resist replacing a cracked screen and its a hard one to prove its from material defect and not your actions.
You may have to swallow this one. Sorry.
