My brother laughed when his sister burped at the dinner table, a common occurrence when root beer was served.
What does the phrase in bold modify above? And what is called?
To my mind, burping is the common occurrence, so does the phrase in bold modify "when his sister ... served"? But if so, what type of phrase is it, i.e. the common occurrence?
Is it an appositive phrase because the phrase might be paraphrased into the following?
His sister burping at the dinner table, a common occurrence when root beer was served, my brother laughed, .