We've seen them go into loads of buildings that haven't collapsed. Now they've gone into one that has. Doesn't seem hugely unreasonable.
How many buildings have you been into in the last three months (it's about three months I'm guessing for the Survivors) that have fallen down since?
Of those buildings that have fallen down in the those last three months, how many have fallen down even on the same day you were in them?
Of those buildings that have fallen down on the day you were in them, how many fell down within an hour of you being in them?
I expect none fell down while you were in them?
And I expect no buildings you've been in over the last ten years have fallen down ever?
Yes, the building they went into was on fire, but the same crazy odds apply - The building was set on fire the exact day/hour they decided to go in, and it fell down the exact minute they were in there

It didn't fall down any of the other nigh on 130,000 other minutes over the last three months...
It's contrived and silly, and most importantly unnecessary... Meaning your brain rings little alarm bells for unbelievability/daftness and drags you further away from the drama (ie: It's all make-believe, rather than actually believable). In my case this alarm bell sounds like a loud groan...
What would have been more believable?:-
- A fire had previously ravaged some of the hospital (over the past 2-3 months), and when trying to access some of the upper floors, it caused the floor to collapse beneath them? So them being there caused it. Anytime they went in there over a month would have resulted in the same outcome.
- A group hoarding in the hospital, and a grenade being used stupidly or something, causing a floor to collapse? Again, so them being there caused it. Anytime they went in there over a month would have resulted in the same outcome.
- Or don't bother with buildings needing to fall down, and concentrate on the drama/characters in these horrible times?