Turns it around when confronted
You raise a missed commitment. Ten minutes later you're the one apologising for your tone.
Rule out first
You really did raise it badly. They have a genuine counter-grievance that has been building. You're both under strain.
What could explain it
DARVO: Deny, attack, then reverse victim and offender. A consistent pattern when held accountable, especially for serious things.
Shame: Accountability sets off shame so fast that attack is the only defence; later, alone, they may admit fault.
Naive realism: Both of you see the facts differently and believe you're objective; it eases with a neutral third party.
Narcissistic injury: Being questioned is the offence; they defend their worth rather than the facts.
Projection: They accuse you of the very thing you raised ('you're the one who doesn't follow through').
Doer and done to (Benjamin): Both of you sincerely feel attacked and the argument loops on who started it. When you name one thing you did wrong, they soften and name their own; if they don't, look at DARVO.
A next move
Raise the next issue in writing, with facts only and no adjectives. Engagement with the facts means it was your delivery; attack means shame or injury; a smooth inversion with you as offender means DARVO.
If it’s you
When I'm confronted, do I answer the point or go for the person?