Exposure of infants used to be common practice, but 97? That's something far more systematic. Lots of births happening all together. A brothel would be one possible explanation, but there must have been plenty of such establishments, and if there aren't any parallels, then it's hard to imagine it was normal brothel practice. They may not have had
effective contaception, though I vaguely remember reading about condoms made of pig
's gut being found at Vindolanda, but there were always abortifacient herbs.