Hard to help without seeing your system.
I guess it is due to support conditions. Probably restraints cause horizontal reaction due to thrust force and it causes bending moments.
I meant horizontal reaction due to arch/frame action.
Ok, I trust there is a roller support, but how can we help if we cant see it and verify? For sure there is some mistake. No constrainst and no loads in structure you descirbed should give simple-supported beam like bending moment.