My DH does this. I turn down things outright that I know will overwhelm him but when we do get invites out with good friends, he will usually say yes and seem to be looking forward to it. Then, almost without fail, the day before the negativity will start. He doesn't want to go, why can't i tell them he is working/ill? What's the 'big deal'? And I end up angry or upset.
Half the time I go on my own and half the time he does come and has a great time.
I am convinced that it's control/anxiety at a rate of 50%50%. He used to be very social, friendly, outgoing and keen to go out and we had a great social life, but he's gone through the mill over the past few years, had his confidence knocked and put on weight. We have talked about it and there is definitely insecurity at play.
However, I am convinced there is also a bit of spite. As in "I find this really stressful so I don't know why you're so fucking happy and not considering how hard it is for me. Therefore I will have a tantrum"
I have started to call him on it every time and don't get as upset or hysterical as I used to. Now, when we accept invitations, I tell him that if he wants to bail, he can phone and explain himself. I am not making excuses for him.
He is getting better.