OK, I heard a discussion about this last night. This is what I got from it, obj IANACE.
There is indeed a convention/tradition that the Lords do not oppose measures coming in from the Commons that are primarily financial. Osborne & Co. are claiming this change IS financial because it's about saving money on government finances and therefore to object to it is unconstitutional.
However some of the lords are arguing a) the requirement to let it through is a convention, it's not law so they can't be held to it and b) The government didn't pass this change as an act which would have required proper debate in the Commons, they're doing it via a piece of paper shuffling called a 'legislative instrument' which doesn't get properly debated in the Commons and which, constitutionally, the Lords do have the right to debate and object to. Furthermore, this is not a piece of financial legislation, it's a piece of welfare legislation.
Basically, Gideon has tried to push this change through parliament as a whole without any messy debate where people might object to it for a variety of reasons, some of which are entirely valid. He managed to do that in the Commons because the PLP decided abstaining was the same as voting against, even though in fact it's the exact opposite. The Lords however, most of whom aren't sympathetic to the Tories, have taken a very dim view of this being railroaded through and have told him to stick it up his official orifice.
So it's to some degree the fact it's a change that some people actually object to, and in some degree it's because Osborne has done this whole thing in a pretty underhand way that's quite disrespectful both to the Commons and Lords, and the Lords have decided to knock him down a peg or two for doing it.
He certainly didn't seem to have calculated just how much his tactics would piss the Lords off. A lot of the press have been portraying Osborne as some sort of political mastermind recently but the fact is this was his first big test of this parliament and he's pretty much blown it.