Not enough information to give you a definitive answer but if I were to guess - it sounds like she owed the previous plan some premium money - you did say it was the previous plan that deducted it - a Medicare Advantage plan or a Medicare Prescription Drug plan Maybe she inadvertently paid them the incorrect amount.
Things like this normally don't happen in a vacuum - she can do a few things to check on this:
1. she can check her mysocialsecurity account to see what date & details show on the deduction
2. she can call Social Security and ask for more details
3. she can call the insurance company to get more info.
Here is just one Medicare publication that could explain some of the instances when amounts have to be reconciled. This may or may not have something to do with her situation - without more details, it is just a guess on my part.