Original: When his friends cut the cord that tied his lawn chair to his Jeep, Walters' lawn chair rose rapidly to a height of about 15000 f/O. At first, he did not dare shoot any balloons, fearing that he might unbalance the load and cause himself to spill out.
Simplified: His friends cut the rope that tied his lawn chair to his Jeep and Walters' lawnchair went up very quickly.