Masteries on Lyss, probably... most likely Cycle of Revenge in the defense tree, giving her a chance of a 15 percent TM boost whenever an ally gets hit by a critical hit, assuming Warmaiden had at least one critical hit on her turn. 

Thanks, that must be it then. I tried twice and both times the same result but masteries do sound plausible. Didnt think someone would go defence mastery on her

Take a good look at her A1... that looks pretty nice to have proccing on counterattacks every once in a while, doesn't it? Cycle of Revenge and Retribution are both great on Lyss, and it's not like you'd much care about offense tree masteries on her. 

Another possible answer to your question is Maulie.

I have seen people run a Lyssandra + Maulie Arena Combo.

When your Warmaiden hit enemy team, It can cause Maulie passive to trigger.

Maulie Passive fills the Turn meter of whole team which can cause Enemy Lyssandra to jump ahead in meter vs. your Kael.

Obviously, I don't know what hero's the enemy team had because you didn't say.

You only said they had a Lyssandra so it could be possible that they had a Maulie.

If they didn't have a Maulie, It might be a Masteries related situation like L9753 said.