the most probable loss at 100 degree celsius is due to water content of the sample, however, if you are using non aqueous solvent, with bp near to 98 or 97 degree, then its probably due to the that solvent.

Secondly, the labeling of the onset and offset are assigned manually by the analyst, so it might be wrongly labelled.

Check the chemical composition of your compounds, and see what is the most volatile component in it.

If it´s a mixture, volatile component may evaporate right at the beginning. So you see sudden mass drop before analysis start.

Or the given mass and mass measured by device do not match? Check if your scale in the device is correctly calibrated.

Thanks Irum Asif and Michal Hegedus for your answers.

Can I consider it under the umbrella of uncertainty or instrumental error which may arise due to false calibration of TGA instrument, because my samples were hydrophobic in nature so can't refer my results to moisture content loss in very beginning?

Try making sample in its purest form. Impurities can degrade at sudden. Also, try 5-10 degree celcius rise rate. Higher rates may show sudden degradation or volatile valorisation

Hydrophobic samples can retain nonpolar solvents that volatilize at low temperatures.

I suggest that you use slow heating rate, 1 or 2 °C. min-1 to monitor the effect of mass loss.

Check the calibration of the thermobalance with the manufacturer's standard weights and the coefficients of the calibration curve.

Thankyou all for your valuable comments.