I think it's because the turret prioritizes the type of target then the target with the least hp.
In this case the type of target is the same: a champion.
In this case Blitzcrank always has more hp (not % wise).
That's why when you dived alongside blitz you took aggro, then kalista, and then back to you. If you waited for the turret to aggro on blitz you'd be fine.
Think of a turret's parameters to be this: First it looks at what type of targets are in the area (champion, normal minion, brawler minion / super minion).
Then attacks the one with the highest priority (normally the highest is a super minion / brawler minion - unless a champion draws turret fire by attacking another champ under turret).
After doing this the turret attacks the unit within the highest priority category (all champions are the same priority + all super minions have the same priority) with the least hp as to remove a unit as quickly as possible.
The bug i really dislike (which i must also say it extremely rare) is when a turret bugs out as you leave it's attack radius (when the turret initiates it's "final" hit) It can decide to double shot you even if you're now outside its range.
Btw tanking the turret is when they intentionally incite the turret's aggro and carry on tanking it. That's why there is usually a gap of time between the person who is gonna tank the turret and the others that follow.
Edit: I forgot to mention that what I'm saying is based on how I've seen turrets work and most probably have a more complicated method of obtaining/choosing a target.
