One reason I'd be a little surprised is that the employer now longer receives full reimbursement for pa , employer NI and pension of furloughed staff so it is costing them money to furlough people and then they are paying agency fees/pay for the agency staff
I guess I can see a situation where the employer wants something quick and simple done, say for instance a pile of stuff moved from A-B which is going to take 2 people 3 days max. Rather than un-furloughing then re-furloughing 2 people (if you can even do that) it's probably less hassle to get to people from an agency to do it?
I am not sure you can refurlough people....also it may be a job that the furloughed staff can't do...that requires a special skill or qualification, maybe that isn't usually needed or isn't usually needed in quantity.