Here are some simple ideas for strange occurrence mysteries with motives and culprits:

1. A small town's water supply suddenly turns bright purple. The culprit is a disgruntled chemist seeking revenge on the mayor who rejected their water treatment proposal.

2. Every night at midnight, all the clocks in a neighborhood reset to 3:33 AM. The culprit is a tech-savvy teenager testing a homemade electromagnetic pulse device.

3. Residents of an apartment building report their houseplants growing at an alarming rate overnight. The culprit is the building's gardener experimenting with a new growth hormone to win a local gardening competition.

4. A series of impossible thefts occur where valuable items vanish from locked rooms. The culprit is a magician using secret passages to steal props for their next big illusion.

5. Strange symbols appear carved into trees throughout a local park. The culprit is an aspiring cryptographer leaving coded messages to attract the attention of a government agency.

6. Pets in a neighborhood suddenly start speaking in human voices. The culprit is a voice actor testing a new sound system that only animals can hear.

7. All the street signs in town are mysteriously rearranged overnight. The culprit is a retired cartographer trying to prove the inefficiency of the current town layout.

8. Residents report their dreams coming true in minor, harmless ways. The culprit is a psychology student conducting a secret study on the power of suggestion.