Can be due to many reasons and without seeing how you did it, it’s difficult to determine however...
Was the TE wood straight in the first place?
Was the TE well supported while the glue dried?
Was it a shrinking type?
Any change in humidity while it was drying?
My procedure sometimes when adding the second TE plank is to remove the structure from the board and re-pin it with a re-positioned jig spar, so that the complete TE area can be flat pinned to the board while drying (and I select straight wood in the first place, which is easy since I mostly scratch build anyway).