Unexpected reaction during acid hydrolysis of cellulose
We miss some data to make a good hypothesis.


The concentration of the $\ce{H_2SO_4}$ solution.
The temperatures.


These are the main two factors that affect cellulose hydrolysis. Usually are used temperature above 200 °C and pressure above 25 MPa. For me its strange that the exothermic reaction between SA and $\ce{NaOH}$ could lead to an increase of temperature such that.  However some studies show that the reaction can occur at lower temperature with concentrated acid.

Another possible solution is the formation of alkali cellulose.