Mine's PAH not petplan so that's probably why!
It's been common for year in cats for the excess to rise with age, it's quite common that after the age if 10/12 that you also have to pay a percentage of the cost of treatment as well as the excess. I suspect that they're now realising that the sorts of owners who would insure their rabbits are the sorts of owners who do actually take them to the vets and get them the treatment they need, and they are therefore having to expand similar criteria simply to make a profit. I have to say I have some sympathy with the price of pet insurance, just thinking back to the amounts that some of us have paid for treatments recently (not least your emergency treatment, byb). The insurance company has to make a profit somehow otherwise they won't stay in business and there won't be pet insurance at all, so it has to balance off the price per customer against the likelihood of claiming and the cost of claims. Some insurers seem to do that in an individual pet basis and it's claim history, others do it on age/postcode, others do it on the average claim value in the preceding three months. I'm sure there are a myriad of other ways it is calculated too.
It does seem random though doesn't it - when my cats renewals came through, Milly's went up £20 after an £800 claim, Geoffrey's went up nearly £100 with no claim. When I asked why, they said that male cats are considered higher risk than females so the premiums go up more over time. I swapped to insure Geoffrey with More Than which came up way cheaper -I hadn't made any claims anyway so it wasn't as if I risked not transferring any pre-existing conditions.