if you know maths, this should be easy.
if a alt coins wants to be valued at lets say $1, then if bitcoin goes from $500 to $700 (40% rise) then the altcoin would change from
0.00200000 to then be 0.00142857 to remain at a $1 value
need i explain more?
This isn't true for instance bitcoin was down to 540 while Betacoin went up to 0.00006000 then bitcoin rised and now Bet is at 0.00001300 a very huge drop.