Slattery's was a knee injury rather than muscular so I'd put him in your 'part and parcel' category. I agree that the injuries to others were muscular and that must come down to strength training and conditioning. Were they ready to play? Have previous injuries not fully cleared up and / or weakened them long-term?
It's not just the eight this year, we had a bad run of injuries last season too and maybe no-one or no one thing is to blame but it's enormously frustrating.