Why does hyperconjugation help for ring cleavage?
This is a bit late, but my instinct tells me you form the 1,3 diradical. The fusion to a cyclopropane seems pretty reasonable. The alkene is formed from a 1,2-hydrogen atom shift (I admit, this is a strange abstraction), but the formation of the alkene (and the fact that this is flash vacuum pyrolysis) make me think this is not completely unreasonable. Since the mechanism involves breaking the H/D bond, you would expect some selectivity 1:1 versus 2:1. The fact that there is essentially no primary KIE observed is consistent with diradical formation as the r.d.s.

The other reaction, I suspect, forms the cyclopentene with the deuterium substitution on at the 3- position. The secondary KIE would be consistent with diradical formation.