Need some answers

I just killed my Immortal call Ground slam Marauder and always after every death I take in Hardcore leagues I look into the reasons why it happened so I would not repeat something like that. My Marauder was level 69 and I was farming docks for exp on my new gems (upgrading build). I died on a lag when opening a strongbox it had mods: guarded by 3 packs of monsters; 37% increased quality... ; contains 6 additional items guarded by rogue exile; ignites you when activated; releases poison clouds.
Before I open any box with this character I:
1. identify the box
2. go get a group of monsters to preform my enduring cry on
3. kill them
4. preform immortal call
5. open box
6. leap strike out of them
7. enduring cry again
8. and start killing them
Now what happened is I got up to point #6 and I had this time a small PC lag, I'm sure it did not take longer then a few seconds and my immortal call takes 13 seconds to run out, so my screen unfreezes and here I am dead just like max 2 second after I opened the box. I was looking around for who might have killed me and all monster where normal, almost all dealt physical damage that I was immune during the lag and should have been immune when it was over, the exile was called Augustina Solaria I googled it and its seems that spectral throw + lesser multiple projectiles what she might have used on me, so shes also physical damage. The only non physical damage might have been was like 5 normal Voidbearers and mods on the chest, can it take down me that fast??? I had 76% fire resist and -30 chaos, so bearers should had not taken almost 3.9 k hp with almost 150 hp regen/sec while I had the small lag not sure how much can a poison cloud deal.
Anyways sad moment my skill three:
http://www.pathofexile.com/passive-skill-tree/AAAAAgEAAdwB5wPuBS0IsQthDc0S4RQgGjgczh2-K1At0jGeMgkzhzWSNug8LT0PPfw__Ed-TeNQR1hjWK9ZbVnzXz9fmGEhZU1sRm6qcmxyqXTtdqx4DXloeu-E2YTvjM-QVZHOkn2Yb58-n8umV6cwqW6sqrXytz677b02voq-p8AawQTEFcS4xPbGrsbYzzLPftX42L3cI-Lq5FHlz-hm6hjvDvZI9zL46_4K_lT-jw==
I am adding screenshots, anyone that has got any idea what might have been missed by me please comment bellow...

Yep strongboxes are rubbish. :-(

I don't know if GGG intended players to have a good chance to die after opening one. Buy I die all the bloody time.

I can't find the official post. But the lag when opening strongboxes with rogue exiles is because the game has to load the cosmetic items they are using like special effects and pets. The post is somewhere on this forum.

Not much to do about that but you can make opening strongboxes much safer by adding additional layers of defensive stuff.

Lightning warp
Smoke mine
Skeleton totem
Freeze totem <- my personal favourite
Quartz flask
Frost walls
Freeze mine
Cold snap
Cwdt+enfeeble
And so on..

It must have been the voidbearers that killed you. When you get focused you die fast even with good resists. I find that arctic armour helps more than resists vs voidbearers. But that skill is not possible to use with every build.

I don't know the mods on the chest but ice nova can kill most charachters in one hit. Same with the explosive mod. If there are corpses nearby the chest will chain detonate all of them like vaal detonate dead. I have been one shotted by that several times with over 4k life.

Thanks for your opinions. I have wrote that the box had these mods and its should be visible on screenshots : guarded by 3 packs of monsters; 37% increased quality... ; contains 6 additional items guarded by rogue exile; ignites you when activated; releases poison clouds.
And for additional defensive stuff I would need a tip guys... I am running immortal call as my main defense against physical damage and CoDT is something I have never used it, any tips? From what I have tried with this char out it was breaking my defensive rotations (enduring cry uses global cd) and i was not ready to immortal call cause it would had casted when there was minimum monsters around...
From what I have read on wiki it looks like the it supports gems till some sort of level (lvl 1 CoDT supports only gems up to 31) correct? What would you recommend for a fighter?

Many people use a CwdT Enduring Cry Immortal call combo.
Although it is not as reliable as a self-cast IC. (Its duration is shorter due to the gem level und it can end just before you take a big hit)

Try to use concersion trap linked to multitrap.
I usually use it twice (=6 traps) before opening strongboxes and just keep staying there.
I hardly ever die because the traps reduce the number of enenies and aggro some other monsters so you are not attacked very often. Very underrated gem IMMO.

If I had to guess I'd say poison + a bit of help from the friendly neighborhood voidbearers.

Chaos damage is not to be taken in stride in merciless, even the poison clouds from the zombies in CoS/Fellshrine can sap your health in seconds. Voidbearers are no joke either.