When bitcoin makes a pump or a dump, the alt varies in BTCprice as well, and thats strange to me. (as your explanation (as far as i understand) states the $$ price must go up and down seems logic as the BTC value varies in $$ value. That makes sense. But why does the BTC value of the alt change so much as well?? Thats double... For instance: when BTC takes a drop of 10% in $$ value, the ALT makes adrop of 10% in $$value (logic), AND a drop of 10% in BTC value as well (unlogic), makes a total devaluation of that coin of 20%..)
Cannot comment on specific instances, but trading goes on all the time. As far as very short timeframes go, there are bids and offers sitting on the exchanges that are hit (often by bots) when certain conditions change.
The round-robin of exchange rates often means that one change starts a cascade of trades. Obviously the more volatile alts move more.
With slightly longer times, then of course there is switching and balancing between coins. It all depends on the potential orders in each alt, which could be any amount.