If someone PMs you asking the reason why you've distrusted them, you could--if you wanted to be polite--tell them the reason, but you're certainly not obligated to. Nor are you obligated to make the reason(s) public for everyone that you've distrusted.
I've had members give me a ~ that made me scratch my head, but I've never gone so far as to send anyone a PM about it. The way I figure it, it's their right to distrust whoever they want without explanation, and I'm not going to be trusted by everyone on this forum. So OP, do whatever you feel like doing as far as this goes. You don't owe anyone an explanation, but that doesn't mean you have to withhold one, especially if they ask for a reason.
OP:
I recently tried to politely ask one person why they placed me on their distrust list only to have that person go on an irrational attack which caused another person to add me to their distrust list.
Not content that person I'd contacted then created a thread about me which has been referenced by yet another random person as an opportunity to put me at the butt of their joke.
Turns out the first person distrusts me because a long while ago I gave trust to someone else. Said person (in the mind of the one giving me distrust) has done some transgression or other. Instead of contacting either of us about their concerns, we were both slapped with distrust.
Cause and effect. I randomly selected one user from a thread we post in - I now have two negatives for my trouble.
OP: If you're an unreasonable sort of person, then you'll go down this path and end up causing unnecessary grief. If you're a reasonable person you'll respond and work through any concerns. I've been contacted recently and worked through some issues, but that's just me.
Good luck navigating this cesspool.