It seems to have been successful but a counter attack resulted in a court enquiry??

It was only partially successful; yes, it did achieve surprise and the opening gains were very impressive, but the forces of British 3rd Army proved unable to effectively follow up that success and roll up the surrounding positions of the Hindenburg Line. In a matter of days, most of the Tanks had broken down or been destroyed, German resistance in the IV Corps sector hampered efforts to establish a flank there, while the Cavalry Corps' divisions were held too far back and received conflicting orders, which meant that they were seriously delayed in reaching the St. Quentin Canal. Even when they did, a tank had already collapsed the only bridge. While it was a tactical success, no serious thought seems to have gione into developing the offensive beyond a vague notion of rolling up the German lines, and this proved impossible anyways.

Aren't counter attacks expected?

The German counter-attack, which retook most of the ground lost and even occupied the original British frontline in some areas, was NOT expected. GHQ (BEF head quarters) intelligence services appear to have been unaware of the movement of German divisions from the Eastern Front into the area, and the German counter-attack, like the British attack, achieved complete surprise.

If you're interested in further reading, I'd recommend Jack Sheldon's German Army at Cambrai, Cambrai 1917: Myth of the first great tank battle by Bryn Hammond, as well as the section on the battle from David Kenyon's [thesis] (https://dspace.lib.cranfield.ac.uk/bitstream/1826/3032/1/D%20Kenyon%20Thesis%20corrected.pdf) on British Cavalry on the Western Front, 1916-18.