The pipes are typically ~1000 psi. That multiplied by the entire inner volume of a rather long pipe could totally make rather a bang...
Which convinietly covers up any explosion signs...
I think it would be hard to destroy an undersea pipe in a way that looked 'natural'. The broken ends will be cut out and brought ashore for failure analysis, and they'll soon find out if your explosives bent the pipe inwards first...
I think your best bet would be to deliberately snag an anchor around it, and then say 'whoops'.
Does it mean there was an explosion?
The pressure in the pipes went from +100 bar to 7 bar. Depending on your definitions you could call that an explosion i guess.