Weak and dropping plant and yellow leaves 2 weeks after topping.

N deficiency -- if not a side effect of what is causing the drooping. N-deficiency symptoms are about as discrete as they can get. Any gnats flying around? could be chewing on roots. Root zone problem of some sort. Most people don't adhere to well-established watering habits, which could also cause drooping or exacerbate leaf symptoms. 1) fully saturate. 2) wait for appropriate dryback and repeat. This simple process should never cause a problem. It's not 'overwatering' to full saturate a medium. It's the proper way to do it. If any droop occurs immediately after an irrigation, it's caused by a less than ideal constiatution. Add more perlite or similar next time. If it's soilless, you should also get a minimum of 10% runoff every time, but the basics remain the same. You can run 20-30% runoff and not "overwater", so it's not anywhere near a risk to do so.

That has got nothing to do with topping.

Toping cant cause it, fact. More like underwatering now, but overwatering before. Magnesium issues because of ph drift. Now plant in lockdown or wrong ph dont let to uptake nutes. Start measuring ph, wise would be to check run off, just learn how to do it right for getting accurate readings.