Equipping a block from the toolbar automatically places it. Flint and steel also ignites something when you equip it from the toolbar.
Linked issues
Comments 2
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This issue was reported previously as MCPE-9545. Comment your experience there!
Confirmed on LG G3 Android 5.0
It doesn't seem to happen every time - except with torches. Selecting water buckets are particularly annoying as they immediately throw water!