to my somewhat untrained eye, it seems as though your shins are departing from vertical very early on in the movement?

Maybe this suggests that the hips / hamstrings aren’t being loaded first in that ‘sitting / reaching back’ action? maybe you’re not squeezing that butt as tightly as possible? 

your shins do remain relatively vertical though which seems good, just that the knees translate forward early.

This might also be linked to the fact that as you mention your toes are up, and it also appears as thought your instep is lifting up too, and you’re weighting your foot towards the outer edge and towards the heel.

This could indicate you need to be centred more over the arch with your weight just in front of your medial malleolus, and screw those hips into the ground more to really keep that stability?

just my initial thoughts.

Maybe try and do them bare foot / in your socks so you can really see what’s going on as the heel, although small, in your trainers will influence your movement pattern.

good job though.