The pressure in these pipes is rather high. Wikipedia [1] writes the nominal for these underwater pipes is 3200 psi. To compare, peak cylinder pressure in a gasoline car at full load is about 1000 psi, large heavy duty diesel engines peak at 2500 psi. A pipe failure at 3200 psi internal pressure might cause an accident similar to an explosion.

Still, you raised a good point. I have no idea how fragile these pipes really are. Apparently, Swedish police is investigating sabotage [2], we might get an answer eventually.

[1] https://en.wikipedia.org/wiki/Nord_Stream#Baltic_Sea_offshor...

[2] https://finance.yahoo.com/news/swedish-police-investigating-...