Law enforcement sources tell us ... cops chose not to give Rocky or his team a heads up. In fact, we're told the arrest was more than a week in the making.

There's a very common procedure LAPD and other agencies employ when they arrest a suspect ... a voluntary surrender. The cops would typically call the suspect's lawyer or other rep and arrange for the suspect to turn himself/herself in without fanfare. That didn't happen here, and we're told Rocky and his team can't understand why ... it's not like he would escape -- and, by the way, his girlfriend is in the 3rd trimester of pregnancy.

Our law enforcement sources say the reason they blindsided Rocky was because they wanted to see if they could find the gun used in the shooting. As we reported, cops believe A$AP shot a man in Hollywood during an argument back in November. The bullet grazed the victim's hand. Cops wanted to find the gun and worried if they gave Rocky a heads up, he might 86 it. That's why they coordinated the arrest and the search of his home so both went down at the same time.

We're told at least 10 cops were waiting for Rocky when his private jet pulled up to the terminal ... and that's when they cuffed him and took him away. Rihanna and Rocky were on their way back from Barbados.

We're also told Rihanna wanted to go by his place Wednesday morning and pick up some of her stuff but cops wouldn't let her in.