Usually, when do the plumbots poke the sim while giving a tune up?

[Summers]

Hello, my sim is level 7 on bot building. I've created a plumbot and he never complains about receiving it. So I thought now he is safe enough to give a tune up for plumbots. But a female plumbot visited my lot, and he opened a new trait chip slot and added 2 trait chips as a present. After that, they became friends. I thought it would be safe enough to give a good tunning but suddenly, she poked him while giving and red minus showed up. I still don't understand why she did even though she had no negative functions. 

[Steellace]

I haven't figured out why this happens either, but I've had it happen even when the skill was up to level 10.  The only reliable way I've found to do maintenance without the chance of the bot getting angry is to have the bot ask instead of the sim.  Since that's not always possible, I just have my sim apologize right away so that their relationship doesn't degrade and try again another time.