Because a margin shortfall arose on an open position — the funds and collateral backing it fell below the margin the exchange requires. The exchange levies a penalty on the shortfall for every day it lasts, and that penalty is passed on to you. It's charged separately from any profit or loss on the trade itself, and it isn't a Rupeezy brokerage charge.
But I had enough margin when I placed the order — why the shortfall?
Margin keeps changing while a position is open, so a gap can open well after a perfectly valid order. Three situations account for most margin penalties on Rupeezy.
An ad-hoc margin change on an MTF or T+5 position
The exchange can revise margin requirements on a stock ad hoc, part-way through the life of your position — not only overnight. When that happens on a stock you hold under MTF, the extra margin becomes payable at T+1. If it isn't covered by then, the gap is a shortfall and attracts the penalty.
T+5 works differently. Only the ad-hoc margin that applies on the trade day (T0) is collectable from you — the exchange's later revisions to that stock's margin aren't collected against an open T+5 position. So on a T+5 holding, this cause is tied to the day you bought, not to the whole time you hold it.
Delivery margin on an ITM stock F&O position
Stock F&O contracts settle by physical delivery. Once a long option position goes in the money (ITM), the exchange starts levying delivery margin on it — applied from the end of the day the position first becomes ITM, and held for as long as it stays ITM into expiry. This catches people who are carrying a long-standing position and don't realise it has crossed into the money, because nothing about the position itself changed — only its moneyness did.
A calendar spread breaking
A calendar spread — legs in two different expiries on the same underlying — gets a margin benefit, because the two legs offset each other. When that spread breaks, the benefit is withdrawn and the remaining leg needs its full standalone margin. If your account can't cover the difference at that moment, you're in shortfall. A spread breaks when the nearer leg expires, or when you close one leg and leave the other open.
One more general cause worth knowing: mark-to-market losses eat into the cash cushion behind a position, so a shortfall can open up on a position you never touched.
The full breakdown, the penalty slabs, and how to avoid it are in What is a margin shortfall, and what is the penalty?
What should I do now?
The penalty accrues per day, so act quickly — add funds to clear the shortfall, or reduce the position to lower the requirement. Meeting margin with shares you already own is also an option: see collateral margin from pledging.