Consecutive is the default if the judge doesn't specify. Some laws require consecutive. Other than that it is up to the judge to specify.

http://criminal.lawyers.com/criminal-law-basics/how-do-multi...

>If a judge wants to order the sentences to run consecutively or concurrently, she must state so specifically in the sentencing order, which is the court document that sets a defendant's sentences. If she doesn't do so, the sentences will run consecutively automatically.