Good question.

No, that's not the reason. I always keep minimum 20% of pool balance liquid, so that should not happen easily.

The negative USD profit is due to DEC price drop. I have many cards that were purchased when dec was near the peg. Now I'm selling those cards at USD loss but still earning more DEC than what I payed initially.

For example:
1$ card would be 1000 dec at peg
And also 0.5$ would be 1000 dec when dec is half of it's peg