They maybe can’t prevent it, but they could leverage BC/Epic/whomever into providing some kind of decent severance for those left out in the cold. In theory.
Their entire minimum revenue guarantee scheme to lure over indies and publishers is apparently extremely expensive for Epic and the store has yet to turn a profit on the whole. Before, they could run it off of Fortnite profits alone. That said, Sweeney has admitted that well is basically running dry and it's why they've been doing layoffs and the like everywhere.
If a company is in trouble, the first thing on the line are the subsidiaries and Bandcamp is a questionable acquisition on the Epic rep sheet, being a music store for a company whose main business is making videogames and selling a gaming engine to people. That means it either goes under or is sold off because it's seen as an irrelevant "aside" subsidiary.
There was no investor pressure.
This may have been a last-minute accelerated deal process where Epic would have all but shuttered Bandcamp if a buyer couldn't be found, before they needed to announce massive layoffs to assuage investors. Songtradr is given an opportunity to see the deal, wants to ensure Bandcamp's survival in its current form (because its demise would hurt the entire ecosystem Songtradr depends on).
But Songtradr isn't given time to put together a transition plan or identify the exact legal path to deal with the union (especially given that they're in an entirely different country with different labor laws!) and not expose themselves to liability. So they announce conditional openness to the acquisition (per the link above, the transaction hasn't closed yet), and this at minimum gives Bandcamp a stay of execution while they identify next steps.
Now, they've identified at least one next step - how much of the current Bandcamp costs they can carry during the transition - and that's 50% of current staff. A tough call to make, but if the alternative was shuttering the service, a very justifiable one.