What caused the damage to the stern?

[Michael-McDonnell]

If you go by implosions as member Jim Currie implied on another thread due to the stern being vertically and going down fast it would cause air pockets and water pressure to build.

Here with the Oceanos sinking completely vertical you notice the suction around the ship as she's going down and steam from it.

 

[Mike Bull2019]

What you call 'steam' is water vapour caused by air being forced out of the ship. This is much more evident in the actual video of this event (and there is no visible 'suction').

 

[Samuel Halpern]

And there is no implosion.