I would like to add some aspects to the anti asic and protection against 51% attack discussion.

More concretely I would like to address the questions:

Is is the upcoming fork the reason for the attack?
In my opinion unlikely but may be. I think the fork and bitmain could be kind of a catalyst.

Was the attack possible because a fork did not take place long time ago?
Yes, definitely!

Why do I think so:
An attack is nothing that happens spontaneously. When you want to attack a network with a double spend attack you have to accumulate the currency before and not too many people noticing it. Also must ensure you got a manipulated node that builds its own chain and so on. This is nothing you do in few minutes, it requires knowledge and preparation. What I meant is: it may be that this as well as the past BTG attack was planned for a long time. Therefore I do not think it was a direct reaction to bitmain or the upcoming fork but it may be that the fork did determine the time point for the attack because once the fork happened the vulnerability is gone or mitigated.

Why that? Well the vulnerability is that the ratio of potential hash rate for the network over the actual net hash is too high. Zcash is the largest coin with Equihash 200.9 and has more then 10 times the net hash rate of ZEN. So it is natural that 10 to 15% of Zcash net hash are avail for rent just on one provider. While this is no problem for the big fish Zcash itself it is one for every small coin that uses the same algorithm. 51% Attack is available for rent! And it will be as long as Zen is not the largest coin of those sharing its algorithm.

Therefore the Zen team should have reacted long time ago - maybe when starting the coin - by forking to another algorithm - one that is unique for Zen! IMAO that is the only way to get rid of such attacks because as long as we stay on this it can happen again and it will happen again if the ratio between the price for doing the attack and the outcome for the attackers is positive. So either they bomb Zen price down to 1$ or the netHash increases high enough that the avail Equi 200.9 hash rate is no problem - then it will also not be one when the ASICS come.

More easy would be a fork to another algorithm. But please: be creative! Why do most now forking coins announce they want to use 144.5? When all do, then still the Zen network hash may be a problem for BTG or BTCP or others. Just the same on a different scale. Why not be more creative and do something unique for the coin? 192.7 has similar requirements regarding memory but already used by a small coin called Zero. Hmm .. but in between 168.6 could be an option.

Or why not pick something that also prevents large mining farms to be build for mining. Already 144.5 causes problems for GPUs with 2G of ram. 150.5 would almost be not possible to mine with 4G GPUs, 156.5 would rule out 8G cards. Going to  162.5 or 168.5 would mean that - except for extremely expensive server GPUs like Tesla V100 32G -  there are no GPUs with large enough memory to mine the coin. The only way to do it is on CPU in RAM (making large farms expensive) or swapping memory from GPU to host computer during the computation to ensure enough memory is on the card to continue. The latter would require GPUs to be connected by fast PCIE connector (so no riser) and to have enough swap space on the system to put the data there.

Of cause that also may rule out many private miners, but in fact it makes the coin non-profitable for huge data centers and huge mining farms that can easily swap their whole hash power quickly to wherever they like making it dangerous again.

So my conclusion: to prevent this happening again it is better to fork as soon as possible and please not to an algorithm already used by any coin with more value then  Zen. Doing this to protect the network from more attacks should be the main priority for the Zen team.