Lightroom export weird lines. Only in exported Image.

In reality, was the sky blue, was it not black? To increase exposure or lets say brightness in components if the photo try local adjustments via adjustment brushes and turn range mask on and avoid including the sky behind the city, behind the fireworks. The mask need no be neat, by turning range on you can disclude the sky.

"Its not that, It happens for 16bit Tiffs also...."

Then the banding is happening in the display path, which is only 8 bits. As WobertC   mentions the settings you've applied are pretty extreme, especially +5 Exposure. Try backing off the controls especially Dehaze and make sure your Export Color Space is set to sRGB to minmize banding.

I will have another 'stab' at an answer-

The development has really 'overcooked' the image! No wonder you see artifacts- even in the Lightroom Develop panel on the raw!

A 5 stop increase in Exposure. wow!

Shadows +100

Texture +63

Clarity +79

Dehaze +62

All will contribute to a breakdown of image details.

Reset and start again.

I am inclined to suggest it is the artifact commonly called 'Banding' as a result of the JPG Bit depth reduction to 8bit with compression.

See- http://regex.info/blog/lightroom-goodies/jpeg-quality

I have seen suggested that introducing some 'Noise' (in Lr) to the blank sky areas can help overcome the banding.

Show us two screen captures -- the image as seen in Lightroom, and the exported image with the wierd lines.