You would get a clearer answer if you specify the amount of bitcoin which are going to be sold. "Huge" can be anywhere from 1000BTC to 1MBTC. The former won't even move the market while the latter will likely result in a substantial drop on the price.
I totally agree with you that if there are no substantial amount of bitcoins sold in the market like the amount you have mentioned then it cannot be called panic selling. Why would there be panic selling in the first place, it is the most unlikely thing to do for a bitcoin stockholder since bitcoin will not have a price deflation.
The most likely reason would be that people bought with the expectation of price increases too close to the halving. People assumed the halving would mean a doubling of the price after the halving, ignoring the fact that markets are forward looking and that if the halving was to double the price, it would have happened slowly and gradually before the halving, not after. And that's exactly what happened. Since last August, the price has more than doubled. (Almost tripled.) People who missed that run up and expected to have big profits buying late in that rally would be worried about loss of capital once the people who bought a long time ago start taking their profits. The race to the exit could hammer the price.
In short, the majority of people don't buy Bitcoin to hold it long term. They buy it to trade for USD profit.