If you're using 8'' zip ties, they're probably too big and that's why they're coming loose. I've always used 4'' zip ties (and I can vouch for the brand Home Depot sells, I've used those before and they sell a HUGE bag of the 4'' ones). Have you seen him getting out of the pen? Does he jump or climb out? At 28'', some rabbits can jump that... but climbing up a corner is a possibility, too. When my lionhead was younger, she repeatedly escaped from a FOUR FOOT TALL x-pen. I was totally stumped!... until one time, I finally walked out of my room and caught her halfway up - she was climbing the corner where the x-pen met the NIC condo. I put a stop to it by zip-typing a sheet of posterboard to the top of the x-pen in that corner.