Sahil Sura
Ansys Employee

Hello Sam Ho, 
Underconstraining the model might lead to convergence failure and also might be the reason for the part to fly off. 
You can specify the appropriate boundary conditions to properly constrain the model.
Understanding the Need to Fully Constrain the Model - Lesson 2 - ANSYS Innovation Courses
If in the case, the model is supposed to be underconstrained, you can take advantage of 'Weak Springs'.
The option can be accessed via the 'Analysis Settings' and you can set the value to the program-controlled or may define an appropriate value.
More on the same can be found in the following reference-
Solver Controls (
One or More Parts May Be Underconstrained (
61.8. Recommendations (

If you want to introduce specific constraints, joints also seem to be beneficial.  Joints (

Hope this helps!