Question about the OverTime battling

Is there an execute when the match goes into OT speed boost or what ever you want to call it? I ask because I've had My units randomly die before When they were 70% health plus against non predator units. In this case it came down to My lvl 2 ww vs his lvl 2 naus (first few rounds or so) and in the final moments of WW vs Nasus even though WW started to make the Ult animation with at least 40% of his HP he suddenly just dropped dead He claimed it was due to His warden Buff but we both had the buff and that doesn't account for his sudden burst of damage. I can't find much detailing what happends other than if you Draw its supposed to end in both parties taking damage.