I gather that when you conjure a weaker monster, it's easier to keep under control, and when you conjure a stronger monster, then it may resent being yanked away from the Feywild (or elemental planes or celestial planes, as appropriate) to do your dirty work.
This is not RAW, but consider as a house-rule: if the tasks you assign are consistent with the motives (and alignment) of the summoned entity - for example, if you summon a water elemental, and all you have it do, is change the course of a river - then at the end of duration, it's annoyed, but not actively hostile, it has no interest in attacking you. It might finish a sufficiently interesting/amusing task - though it might also finish the task in a different way than you intended. For example, if you summon a celestial, and you tell it to fight some infernal demons, and you lose control, then it might still enjoy fighting demons. It might then go hunting for imps... and destroy the imp familiars of a few local warlocks... and those warlocks might blame you.