It crazy why that happens, i think it because there is so many amatuers in the space, Bitcoin prices rises so they want ot buy at cheap price so sell all there altcoins, then go back to invest that money into alt coins, it makes no sense what so ever... maybe you have a different view why this happens.
One thing for certain, its sooo annoying.
Yes, maybe some exchanges should add more USD pairs, so amateurs can stop trading only BTC pairs (like me). It will change a lot of things.
I mainly hodl my alts, not using BTC volatility to take profit on alts, not my kind of trading, just hoping not being crushed by a btc rally, even if I think some alts will be in phase with BTC at some point.