any change in diet can cause runny poo though it should stabalize after a few days. Always sitting in his hide and pooping there will result in caked poop on your cavy's bottom.
If this is suddenly new behavior in an other wise clean cavy with solid poops, then I would address the changes that have been made recently. Adding timothy hay as part of their diet could result in some watery poop but should go away in a few days, unless you are feeding poor quality hay or hay that is starting to rot. Bad hay can cause health issues.
If your cavy has been having this poop problem before giving timothy hay or continues to have it after a few days of feeding clean timothy hay, then I suspect an internal problem, such as bacterial infection, parasites, etc. A vet trip will be necessary.
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PHLdyPayne