I'm familiar with the phenomenon, as my wife is a linguist who did her master's thesis on the phonology of a small language spoken by about 7000 people: many of the kids don't want to learn it, and just want to learn the majority language of the country since that's what they have to use in school. But I didn't think that could be the explanation for a 25% decline in ten years: new people may not be learning the language, but the only way people stop speaking their mother tongue is if they immigrate to a new country and fully adapt to it (happens to a few people, usually who immigrated as children) or if they die (by far the most common reason for language-use decline: the old people are dying and the young people aren't learning it). If the decline was a couple hundred thousand that would be the outside limit of probability, as far as I know.

More likely, in my opinion, is that both are happening: yes, the language is declining, but either the earlier census overcounted speakers (e.g. counting children as speaking it when they weren't actually learning it) or else the later census undercounted speakers; either way the language decline would look larger than it actually is. Given that Ethnologue (https://www.ethnologue.com/language/hoc/) rates the language vitality as "Stable" — "The language is not being sustained by formal institutions, but it is still the norm in the home and community that all children learn and use the language" — and they usually know what they're talking about, I suspect the language decline isn't that fast and a census counting mistake is a more likely explanation for the discrepancy over ten years.