Some laptops have a mechanical lid switch (instead of a magnetic one) that easily triggers when some pressure is applied to the lid. I have the same problem with... a Thinkpad. Luckily, under Linux, you can disable wakeup through the lid switch:

    # cat /proc/acpi/wakeup 
    Device	S-state	  Status   Sysfs node
    LID	  S4	*enabled   platform:PNP0C0D:00
    ...

    # echo "LID" > /proc/acpi/wakeup

    # cat /proc/acpi/wakeup 
    Device	S-state	  Status   Sysfs node
    LID	  S4	*disabled  platform:PNP0C0D:00
    ...

It's not a one-shot setting though, as the desktop environments reset it before suspending. As far as I can remember, I put a script in /usr/lib64/pm-utils/sleep.d to make it permanent.