Contracting 〜ているのだ to

Grammar: Colloquial and Casual Speech

Casual speech can contract 〜ているのだ to 〜てんだ. First 〜ている becomes 〜てる, then explanatory の becomes ん: 〜てるんだ. Fast speech compresses the remaining る, producing 〜てんだ.

Examples

Why are you silent with 〜てんだ

Manga panel from 三ツ星カラーズ showing example of Why are you silent with 〜てんだ.
三ツ星カラーズ » Volume 6 » Page 23

Being the first to arrive at the clubhouse after school, Yui decides to change her hairstyle and see how long it takes the others to notice when they arrive. Sacchan arrives next and starts up a conversation. Yui simply sits there without saying anything.

さっちゃん:
「なんで黙(だま)ってんだ!」
“Why are you being silent!”

You've been found out with 〜てんだ

Manga panel from 名探偵コナン showing example of You've been found out with 〜てんだ.
名探偵コナン » Volume 1 » Page 8

Shin’ichi reveals that the murderer escaped out the window and onto the roof, naming the wheelchair-bound husband as the culprit. The husband points his cast-covered leg, but Shin-ichi knows better.

新一(しんいち):
「もう、ネタはバレてんだ!!」
“Your deceit has already been found out!”
メイド:
「だ、だんな様(さま)、足(あし)が…」
“Master, your leg...”