What Causes Bulging in The Fabric While Tufting?

The canvas can bow due to too much close thread being shot in one section.

It's happened to me several times, I have someone to help me too. 

Next job I'll try to stay slightly further away. It bent especially on really small jobs, I'm talking about 25cm x 30cm. It is clear that the textures are very dense.

I should have lightened up. But by making mistakes you learn!