Step 1: List what cold cracking actually needs to happen.
Cold cracking, sometimes called hydrogen induced or delayed cracking, needs three things acting together, a hard and brittle microstructure, diffusible hydrogen trapped inside that structure, and residual tensile stress pulling on the joint after it cools.
Step 2: Trace where the brittle microstructure in a high strength steel weld comes from.
High strength steels are quite hardenable, so when the heat affected zone is rapidly heated by the arc and then cools quickly afterward, this fast cooling behaves like a quench, and the austenite present at high temperature does not have time to transform gently, it flips instead into hard, brittle martensite.
Step 3: Connect that brittle phase to the cracking behaviour.
This freshly formed martensite is exactly the susceptible, crack prone microstructure that hydrogen needs to attack, hydrogen diffusing into it lowers the stress needed to start a crack dramatically. So it is this martensite formation in the heat affected zone that sets the stage for cold cracking in high strength steel welds.
\[ \boxed{\text{martensite formation}} \]