[bluGill]

They are forced to because the lender of the stocks is afraid that they don't have enough money to buy back if things go even higher. If you can prove you have enough money at current prices then you are not forced to sell. GME is not a big company, even at current prices there are several funds that could afford to have a significant short interest from the bottom without getting a call.

edit: originially I said they could carry 200% short, but that is not true. The biggest funds I can find could alone have shorted 30% of a GME at $3 and be okay at $300, but going much above that price or percentage of GME shorted would be a problem.