To be honest I think he was nervous about tipping it over and made his situation worse.
A lot of newby's have an uneasy beginning in kayaks and over-compensate for any rocking that may occur, often to a point of creating a rhythm of unbalances that make it feel worse than it is.
That kayak he was in shouldn't have felt unstable esp with 100kgs above it.
Whilst that kayak is entry level, it should be fairly comfortable to be on the water with.
Only thing i can wonder about was his posture in the kayak......were his knees up in the air or basically down in the kayak body? Raised knees raises the centre of gravity and can exaggerate the lack of stability.