can happen if you don't adapt to the riding characteristics of the mantra. if you hit the front brake on a decline it tends to "scissor" i.e. the rear wheel continues to move forward and the seat goes up. that's more pronounced if the rear damper has a lot of sag and the fork is set up relatively soft. if you adjust the suspension within reasonable ranges, first hit the rear brake, lift your butt out of the saddle a bit during braking and move the weight behind the saddle it is no problem. as with all URT designs the rear suspension is not very active when the rider is out of the saddle but that's mostly during climbing and then the mantra behaves more like a hard tail (which i like).
I've always wanted a Mantra, bumped into an old guy riding one a few weeks ago. I was on a group road ride and came across him 'walking' his dog on a quiet country lane. The bike was the pro model and in minty condition from what I could see, he must have been in his early seventies.
My mates had one for 10yrs now. Its gone down the pecking order a bit, behind the Maverick, but he still rates the design.
As Carsten has said it can have bad 'sting-bug' tendencies on steep stuff or under heavy braking. However for all day epics or climbing it is a brilliant bit of kit.
The original was also designed with a rigid fork, but the PR department said it wouldn't sell so they fitted a short travel front fork