Google search shows that "to die of/from injuries" is a more common phrase than "to die of/from injury". Is there any explanation for this pluralisation? Does it mean it is wrong to say the latter?

Such things are often idiomatic. However, here are a few quick thoughts.

If you die, often you have more than one injury.

If it were one specific injury, I'd use an article, eg He died of an ijnury.

But really, if it were just one injury, I'd probably describe it more specifically,

eg He died from a skull fracture.

Both 'of' and 'from' are OK, but we  also often say it in other ways,

eg He died because of . . . 

eg His death was caused by . . .

Clive