Road bike recommendations

Took the BMC TMR01 for some pretty miserable laps of Regents Park tonight.

The 58 that they ordered for me arrived with a broken seat tube so I rode the 56.

Interestingly whenever I gave it some power I slid forward until I was perched on the very nose of the saddle.

Who can tell me why? Same saddle (Arione) as my other bikes (which this does not happen on).

Do you have a longer stem than normal and are pulling hard on the bars when pushing? Seems strange.