Now it's possible that you missplayed i dont know for sure, but think of this, If he ulted you during your lightning rush he will steal more armor and mr (the amount he steals is determined when he casts subjugate) So if you used lightning rush you get bonus armor and MR for 4 seconds, but if he subjugates you during that time , He will calculate how much he is going to steal, HOWEVER after 4 seconds that bonus armor and MR can run out for kennen.
So lets say you are 50 armor kennen and you use a rank 5 lighting rush to go up to 100 armor, Then trundle ults you, He will eventually steal 40% of that 100, in other words 40 armor is what he will steal, so he could drop you to 60 basically, but the lighting rush bonus can run out, making your armor drop to 10 FUCKING 10. in other words if you go hard on him and Use lighting rush to start a fight against him, he can turn it around on you very hard.