Pushing a bit hard can do that, but so can a seat that's too high and/or too far rearward (even by just 5 mm). The bar being too low and/or too far from the seat can do that as well. (Again, just 5 - 10 mm can make all the difference in the world.) PS: just because you've had a "pro" fit doesn't mean the position is perfectly ideal for your anatomy, flexibility, body quirks, etc. I found that out the VERY long way . . .