They must have a weak/invalid defence.
Perhaps the opposite.
Perhaps the prospects of success are so much in favour of the defendants in the principal matter
that they are seeking some confidence that their costs can be paid.
We don't have nearly enough to even make a guess.
For example, we don't even know exactly who the defendant is,
the nature of the claim, nor the statutory basis upon which it's being brought,
nor even in what venue.
All of which is relevant long before we dare speculate about discretionary interlocutory costs.