The one to the far right is cylinder 1 - so its your 1st plug
If theyre the original wires, the end may be worn out - causing it to be loose and pop off. If theyre aftermarket wires...they may be shitty quality
Your plugs may be loose, causing pressure to pop the boot off (unlikely).
Your plugs may also be damaged.

When is the last time you did replaced plugs and wires?