Bitcoin increases, altcoin prices go up? I think you are wrong, when bitcoin prices go up,
altcoins tend to dump, and if bitcoin decreases then altcoins will also dump,
this is the relationship between bitcoin and altcoin, unless bitcoin is sideways then altcoins usually will try to go up.
It's not wrong at all. Sometimes it's really happening but it doesn't mean altcoin can go to the opposite thing. Bitcoin bearish market will always trigger the altcoin market to be dumped and this is absolute. An altcoin is not always getting dumped when bitcoin was pumping.
Someone already explained the possible scenarios that could happen with bitcoin and altcoins. The trend is also playing the main roles in this case.