Why do champs for some reason become tanks for no reason?

I physically have no idea, the amount of times a random ADC, jungler or other would some how become the tankiest thing in the game for no apparent reason, E.g. I had a game where I had 3 dmg items (db, yommus's and edge of night) and kayn had dusk blade and enchantment warrior, nothing else and he would take absolutely 0 dmg?

I've had familiar experiences with this stupid crap (caitlyn having 0 items and taking 30 dmg only?) and I feel its just riot pulling pranks on me or something stupid like that.