the trouble is ...; to make matters worse; troublingly
Fixed expression used at the start of a sentence to introduce an unfortunate or problematic situation. Often followed by an explanation of the trouble.
The trouble is, I left my wallet at home.
To make matters worse, the rain suddenly got heavier.
Variant with 事 instead of こと; slightly less common.
Mixed kana-kanji variant.
From 厄介 (trouble, burden) + な (attributive form of copula) + こと (abstract thing) + に (adverbial particle). Literally 'into a troublesome thing', it became a fixed adverbial phrase.