Using 4.4 build m365

Had 2 outputs running at 1600 x 1200.

4 layers running movies - AIC codec mid res.

1 layer running a phoenix sdi input.

All layers suddenly froze. no movement on any layer including live capture

Brought faders down and the layers faded out ok.

I switched my local monitor on the matrix to see output locally on that machine and brought faders back and everything was ok.

I'm just curious as to why!! Any ideas anyone?