Maybe she felt like she couldn't hide them inside any longer because she was vulnerable to a Search Warrant.
And maybe she felt like she was always being watched outside too, by someone she had not met.
Bottom line is that she didn't feel there was any real evidence on which to hang a conviction. She decided that her best play was to give Columbo what he was determined to find, so that she wouldn't be caught trying to hide them or dispose of them, which would be self-incriminating.
There are several other suspects, such as Ward Fowler in Fade in to Murder, and even way back to Dr. Ray Fleming in Prescription Murder, that pretty much dare Columbo to find some incriminating evidence.
She screwed up in the beginning by trying to write the script for the murder and blurted out that it was an accident... Columbo's response puts her on the ropes; the rest of the episode is good entertainment though.