Based on:
Prevalence of osteoporosis and vertebral fractures in acromegalic patients -- Giuseppina Padova,1 Graziella Borzì,1 Laura Incorvaia,1 Guido Siciliano,2 Valentina Migliorino,1 Mario Vetri,1 and Patrizia Tita1
They say:
"A chronic GH and IGF-1 excess produces an increased bone turnover, reflected in an increase of bone formation and resorption markers; bone resorption markers are disproportionately increased respect bone formation markers and their increase could reflect the degree of bone loss often observed."
And btw, nowhere in the article were they any more specific than that. If anyone wants to delve into the actual mechanism, I'd be more than happy to hear as well.