I'm assuming that the Alexandria was set upon by many Shivans, much like the Trinity. Although the Alexandria managed to hold out for a while, it became apparent that it would not survive the engagement, so everyone made a mad dash for the Grall, which was docked to the cruiser, but only 17 people, including Snipes, made it before the Alexandria's reactor went critical. My guess is that the Grall tried to depart straight away, but the explosion of the Alexandria jarred its engines, so it was probably only capable of making a Hermes-like jump to a random point in the nebula before the engines died.
Wasn't there a discussion quite some time ago concerning which class of cruiser the Alexandria was?