I simply don't see the reason why it can't happen.
Bitcoin dominance swinging lower than 70% is actually good for alts, given the fact that it implies that the alt market is currently outperforming BTC because of the fact that BTC dominance generally resides in the 70-80% region over the long term.
Only when BTC dominance is high does it suggest that participants seek less risk in the market and divert their trades from alts into BTC. Not the other way aronud.