Gibbs and Pakenham where specificly noted by people around them as being cut down by grape. I can find no reference to what got Keane. So 2 of the 3 seniors went down to grape. But why? As you say it has no respect for rank.
Where were those Officers? Big question, but we have a probable answer. Right up in the front rank leading things like they did in most other fights. So, it is not unusual for there to be high officer casualties in that situation. Visibility was poor that morning, and blackpowder smoke with a bit of fog is very hard to see through. Not likely that many could have picked out officers to target them. Sure, some probably where, but not many. And particularly BN command and below was hard to tell the differeence.
Remember as well that they where in colums, not the typical line you think of brits as using. Again that puts a higher percentage of offocers up front. look at some of the loss figures for Napoleons Grand Armee and you will see disproportionate losses among officers. Price you pay for follow me leadership. It can win a lot of battles, but sometimes you just crap out. Doesnt require riflemen carefully picking them off, just luck of the draw.