Before your Mom passed you had already recognized, her passing would be painful and traumatic for you. Also, you knew it was coming, and had time to steel yourself, build some emotional walls to protect yourself from the pain.
With your Auntie, you had not seen her in a couple of decades, so were hardly expecting her passing to be terribly traumatic to you. In addition, you had no warning, or time for protective measures like shutting yourself down emotionally to avoid feeling the pain of loss.
As you can see, you can hold at bay, for a time, the pain and loss you don’t want to face. But sooner or later you will experience something traumatic which will catch you unprepared. When that happens it can trigger a cascade of all of the suppressed emotions you’ve been trying to hide from.
Suddenly you are reacting out of all proportion to the trigger, and you recognize it and find it unsettling as it indicates you really can’t control your feelings, like you attempted. We are not machines, that can turn our emotions on or off, or just ignore them as and when we choose.