why alts prices are linked to btc price?

sample - chainlink price and btc price


sample of price of btc and link:
1 hour ticks

October, 2nd - October 11th

compare big price up and down  movement periods

btc price chart


link price chart


i dont see in description of LINK any info that bind its price to btc - but i see this at picture

why are   they same?

usdt tether market cap grows from 4,6 billions to 10,800 billions from March 2020

For last week - when btc jumped from 9 000 to 11 000 - tether issued 800 millions usdt

Do you think tether really has fiat for backing this usdt? 

at binance from March, 12 up to now (April, 11) someone bought bitcoin and spent 600 mln usd to keep bitcoin at price from 7,200  up to current 6,800

all people there sell bitcoin, and someone bought all this btc, total cost of purchases are about 600 mln usd 
Delta  between btc that was sold  and was bought - about 100 000 btc

any ideas - who, why?