The issue with the way Icefrog approaches the balancing of heroes is that it is nigh impossible to predict how impactful all the changes are for each and every hero/item. Look at the discopony, he wasn't really picked except as a support in a couple of sd+lesh strats, and a couple of SEA teams ran him as a core before 6.84 hit. He got buffed the 3 previous patches, was already in a good spot before, but it takes a while for good heroes to actually get noticed.
Look what happened to sniper @DAC, the patch was out for months and only VG actually picked him up during the tournament which was what got them to the finals. It's the same for Leshrac: He already was a good hero before the patch came, the new items only amplify this power and now he's at a 100% ban stage.
If you ask me, he's actually quite overrated. There are a couple of pretty good picks that work well against him, bounty/tusk to gank him early or phoenix since lesh sucks at destroying the egg to name a few.
Edit: Leshrac isn't necessarily broken, but he wins pretty much every single midlane. Lightning storm got buffed 4 times in a row now (manacost reduced, castrange increased, damage increased, cooldown reduced, it slows now), his base movespeed is pretty high (also got buffed), and his ultimate is something you actually skill now (the manacost reduction one or two patches ago was a huge buff).
What made him rise to the top imo, is that two of the new items are incredible on him: Octarine, for obvious reasons, and glimmer cape aswell. Having an invisible discohorse chase you down without even having to cast a spell is just stupid. He doesn't require 2 mana items nowadays to be able to use all of his spells constantly, which was the case just a year ago.