Double spending is just convincing the network you've sent coins to multiple places, and then undoing all of the transactions that were 'paying' for something (in this case paying for credits at an exchange).

The actual details of what they were doing matters and I don't know them, but it's almost certainly simpler to chain double spends together, one after the other, than to try and do three or more transactions concurrently then reverse most of them.

This achieves the same thing as a triple or quadruple spend but only necessitates reversing one transaction at a time.