It might be strange to think of reserves as constituting a liability. After all, reserves are supposed to be a good thing. If we have reserves of money, then we have money. Shouldn't the presence of money be a boon? Shouldn't saving money be an asset?
You'd think so, but it is, nonetheless, a liability. If we think more about it, we should understand why these reserves amount to a liability.
Perhaps if we worded it differently, it'd be more clear. It's not that the reserves are a liability right now. Rather, the reserves are held in the anticipation of future liabilities.
Any given company faces a number of unexpected and unpredictable situations. Reserves help a company deal with what might come its way. For example, a global pandemic might come along making it hard for a company to continue to function and pay its employees. If that company has reserves, it can use them to keep operating and to compensate its workers.
Another way to think about reserves is in the context of a bank. A bank is required to have a certain amount of money at all times. Again, we should think of this not as an asset but as a liability. The money is not for the banks, it's for their potential customers who inevitably come along and take out their money from them.
Imagine going to a bank to get your money and your bank telling you, "Sorry, we don't have your money right now."
That's not acceptable. A bank is responsible—"liable"—for your money, which is why they keep reserves.