I’m sure most of you have noticed that during the Trench run on Midnight (before the Infinity punches a hole in the Didact’s hull) his rank appears as Captain, opposed to Commander like it was for the rest of the game. So, what happened to Del Rio? Did he die somewhere? Mutiny? Foreshadowing by a malfunctioning Cortana?
Didn’t they say he got degraded because he left Chief behind or something like that?
Lasky mentions something about FLEETCOM not being happy with Del Rio’s performance at Requiem (this happens as you are flying through the Didacts ship). I imagine he was re-assigned or just court marshalled (i think thats the correct term?) 
Lasky said Fleetcom didn’t take kindly to him leaving Chief behind on Requiem, so they removed him from command. Lasky took over.
From http://halo.wikia.com/wiki/Andrew_Del_Rio
“Acording to Lasky, FLEETCOM wasn’t pleased when he arrived back at Earth without John-117 so he was forced to step down as Captain and was replaced by Lasky. His demotion was also likely caused by the attack on New Phoenix, which could have been prevented if he had listened to the Master Chief.”
Aahhhh, cheers guys! I must’ve missed that dialogue.
Side note if your interested. Del Rio was never right for the job in the first place. He was placed there essentially because on paper he was right for the job, however in reality he was not qualified for the position. He lacked what it takes to captain something as epic as the infinity, and lacked the leadership etc a good captain should have. None of the crew really respected him (most notable in the game when Palmer doesnt even attempt to arrest Chief under a direct order), and his position was always only going to be temporary. ONI had Lasky there ready to step up the minute Del Rio messed up, and in reality Lasky was damn near running the show before his promotion, answering Del Rio only because he had to. (All of this explained in the Thursday War, in greater detail than my brief summary lol)