The first 600-proposal was indeed bad for LD, I don't know, how the Tories wanted to come through with it in Parliament. The second version then was - what coincidence! - clearly better for the LDs.
I thought the Lib Dems voted against the new boundaries because Cameron failed to get his fellow Tories to accept House Of Lords reform.
That was their public reason, and I don't doubt it was part of it, but I suspect the realisation that the new boundaries were likely to be troublesome for them, and for their leader in particular, may have had something to do with it too.