The screen becomes jumpy, touch input unresponsive, and camera movement erratic. Typically this issue occurs in Endless mode games.
Depending on which device you have, you are likely hitting the performance limitations of your hardware. During intense games with large numbers of towers and enemies on the screen you may eventually encounter such issues. One tip for improving performance is to set your Graphics setting to "Fast" in the Options menu of TowerMadness. Another suggestion is to try rebooting your device before playing in order to clear up as much available memory and close any apps which may be running in the background.