Well if you check the price history for ethereum you'll find that before March 11 the price range was in $20 and then in the rest of March and April the price was more than the double. The spike got up from $20 to $50 and stayed in $40 - $45 for some time.
I'm not sure if this is the direct reason or not because nothing else makes sense to me. Looking at the difficult at that time, there was no difficulty drop or anything similar so the only thing that would just more earnings would be the price increase of it while btc stayed almost same at the time with not that huge diff. increase (in the range).
Yes, the main reason is because of the price of ETH rised within the period. But why do you ask so late, it was 6 months ago...
