If I have followed things correctly Gail lied to David that Richard's birthday was coming up - to set him up - and the card duly arrived.
David thinks that Phil know's about Richard's supposed birthday and is pinning it on him but Gail knows that Phil knows nothing about all this so now Gail knows that it's David sending the cards.
He is very very angry with her I think because if I remember correctly when Phil was interviewing them for his thesis about Hillman David said he was angry that his mother had brought such a psycho into their lives.
In my opinions there are too many loons in the prog now and they are relying too much on disturbed characters for their storylines.