2 possibilities:
1st: When alt-tabbing, due to the way keyclone works, it treats it as just plain tabbing and sends a Tab to your alts, possibly changing their targets
2nd: Yer using the macro too soon, as oftentimes it takes a full second for the Focus' target to show up on the alts screen due to server lag/time taken to send the update to you, it's not quite instant, and your lock's targeting a mob the hunter targeted a split-second before your current target, or if no target is obtained, when using the pet attack it'll grab the nearest and/or random nearby mob and target it instead.