I can think of several reasons why a coach might choose to substitute in this manner, and only one of them is to intentionally delay the game.
Just as some officials (not naming any names
) like to deride coaches for not understanding the rules of the game, this could easily be flipped around to say that some officials could be ripped for not understanding the strategy of the game. Each substitute for either team can provide opportunities to take advantage of matchups, ball pressure, foul situation, etc. I could at sometimes be guilty of what you described. Figure out the matchup and substitution I want to make, send the sub to the table, then realize I don't have a good shooter in to break their zone, so I send a second sub to get a shooter in. Oops, now I have no ball-handler. Need to sub again.
This can be even more so if the opponents sub during this sequence as well. Depending on their personnel, I can sometimes anticipate their strategy and counter it with substitutions of my own.
Don't always assume the coach's actions are just to pi$$ you off.