The wound fitted to the wrong
The verse pairs two words for wrongdoing with two kinds of bodily harm — and the pairing is not random. 'Pierced' answers 'transgressions'; 'crushed' answers 'iniquities.' The outward wound meets the outward act; the inward destruction meets the inward twist.
Hebrew has more than one word for moral failure, and this verse uses two of them deliberately. A 'transgression' is a crossing over — a visible act, a line passed. An 'iniquity' reaches further inward: what is bent or warped in the person, the disposition beneath the deed.
The verse gives each its own wound.
'Pierced' is a surface broken from outside — the kind of damage that can be pointed to. 'Crushed' is something collapsing from within. A pierced thing still holds its shape. A crushed thing does not.
If the pairing is intentional — transgression met by piercing, iniquity met by crushing — then the verse is doing something more exact than listing two sins and two sufferings in parallel. It is matching the nature of the harm to the nature of the wrong. The outward act draws the outward wound; the inner corruption draws the destruction that goes all the way through.