Maybe the emulsion was fogged after the stop bath, someone using the enlarger while she/he carried the material from one tray to the other. Also as hexavalent said liquid emulsion might hold more developer and the stop bath was not long and or strong enough to stop the development process. Wood holds probably even more chemistry than paper. You might wanna try the Rollei/Maco emulsion hardener it's put into the developer instead of the fixingbath. Use the stopbath for twice the recommended time for normal paper (might attack the emulsion so pre-hardening is recommended)
Again good luck.