Copper, I have been in a similar situation with my husband.
We were at a 'children-welcome' adult party with our oldest son, then 6 years old. The couple holding the party were good friends of ours, with a child the same age as my son. It was time to go and I was waiting in the car, then became dimly aware of some commotion. My husband appeared looking daggers, along with my son, crying in great pain. He had just caught his finger hard in a door. My husband (who is a very proficient first-aider) rushed him to the nearest casualty department. He feared our son had suffered some bone damage and wanted his finger Xrayed. He had not, but the wound was very nasty. The day after, I received some angry calls from our hosts. Apparently my husband had totally lost it with them, because they were trying to reassure him that it wasn't so serious, and told him he was overreacting by going to casualty. One of their guests was a nurse, apparently, and felt my son, thought shocked, was OK. The couple were very cross at my husband's rudeness to them in front of their guests, and felt they were owed an apology.
I said I'd certainly pass on the message to my husband, and tell him how upset he'd made our friends, but that was all I could do. I told them, I wasn't there to witness the crisis, and most importantly, I wasn't my husband's keeper, so an apology from me would be worthless anyway. I was sorry the whole situation had occurred but I was simply not going to take sides in this. I said if they felt like seeing me alone in future, that would be OK with me and with my husband. This is in fact what happened for a while ( now things are all right between us all again - time heals etc etc). Every time the conversation swung round to this incident, as it did a few times, or they cast apsersions on my husband, I changed the subject.
Don't know if this applies to your situation, copper, but could you have a word with the PTA people to make it clear that you and your husband are entirely different people. You can understand their feelings, and you certainly don't condone your husband's outburst, but neither can you aplogise for it, and you don't expect to discuss it with them further.