As for the reason, well, agencies are using extensions to juke the stats, so its not really surprising that requesters are objecting to that. And as the Ombudsman notes, complaints are an essential part of making the system work and ensuring that decisions are lawful and reasonable:
Mr Boshier says there may be a number of reasons why information requests are refused and requesters were doing the right thing by complaining to the Chief Ombudsman if they were unhappy with the response.So, if any agency extends your OIA request for "consultations", ask them immediately "who are you consulting and why could it not be done within the statutory timeframes?". And if you don't like their answer, or they don't respond at all, then I've written a guide on how to make extension complaints here.