That's the effect of CPUs/GPUs now trying to find at ASIC difficulty the first block of the HF chain. I assume 170000 will be the first HF block (not sure since other coins count differently and 170001 would be the first HF block there).

If 170000 would still be on the common chain and 170001 be the HF block, then the chain being stuck at 169999 would be an attack. Someone controlling most ASICs could have switched them off and the chain would therefore not be able to reach the HF block. But the attacker would have done that earlier, not one block before the HF, for maximum damage. I therefore assume it is rather the above scenario.

EDIT: Block 170000 has been mined.