Knock, knock. Race condition.
Q: Knock, knock.
A: Who's t-
Q: Race condition.
This joke relies on the listener knowing how a normal knock knock joke goes, as well as to know what a race condition is.
Normally knock knock jokes begin "Q: Knock knock, A: Who's there?", but this joke unexpectedly circumvents that standard form and has the joke teller cut off the listener mid sentence. There is another joke that does this (which I have already explained) about an interrupting cow.
The reason why this makes sense is that a race condition is when multiple things are happening at one time in a computer, and they will have different results based on when they occur relative to each other. The programmer either has to fix it to guarantee that they happen in the right order, or perhaps might just hope that they come out in the right order.
So, the critical element of this joke is that it circumvents the standard Knock knock joke form.